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Description
The IS-4, also known as the Object 701, was a Soviet heavy tank that started development in 1943 and began production in 1946. Derived from the IS-2 and part of the IS tank family, the IS-4 featured a longer hull and increased armor.The IS tank, released in 1943, had 120 mm of front armour, compared to its predecessor's 75, in roughly the same configuration. However, while the KV's 75 mm rendered it nearly invincible against all modern anti-tank guns, the IS did not have such luxury. Work on an IS tank with superior armour began during that same year.Object 701 was a lofty goal indeed. It was designed to be impenetrable to the most powerful anti-tank guns of the day, the D-25 and KwK 43. As we've seen , the tank eventually achieved that objective. However, early prototypes of the hull had a couple of weaknesses.Due to the spherical shape of the sight cover, a hit at a perpendicular angle, and therefore penetration, is possible at various angles. The rolled roof of the tank can be penetrated by 88 mm shells
The cover of the driver's observation device can be penetrated by 88 mm shells (see photo #20). The factory used 8S steel, which cannot be properly conditioned when over 75 mm thick. Kirov factory representatives insist that use of this grade of steel was unavoidable.

The turret platform roof is penetrated by the shockwave of an HE-fragmentation 122 mm shell."A lucky hit to the welding seam can penetrate the hull side, otherwise it performs very well. The tank's armour proves worthy, but the design of the hull needs some work.

That was Object 701 #1. and had improved welding seams and higher hardening of the turret armour. The test results greatly improved as well: "The armour of the Object 701 tank provides complete protection from 75 and 88 mm shells with muzzle velocity of up to 1000 m/s at any distance, in an arc of +/- 60 degrees from the front of the hull and +/- 30 degrees from the front of the turret." As a result of these trials, the hull and turret designs of the Object 701 were accepted. Object 701 #5 and #6 later had the front plate increased to 140 mm to counter potential 105 and 128 mm guns.

The drivetrain needed some work, though. Object 701 #4 received an improved transmission, which was further improved in #5 and #6 prototypes.

As for the armament, the gun was the same old D-25 that has proven itself against the toughest German armour.

"According to your requests, the Chebarkul GAU proving grounds tested the 122 mm tank gun model 1943 (D-25) in the new heavy tank (object 701), developed by the Kirov factory, between May 11th and 15th of 1945. The 122 mm tank gun trialled in May of 1945 was produced in April-May of 1944. After that, the gun was substantially modified and improved. Most importantly, the gun's semi-automatics were improved, removing the possibility of breaking the breech opening roller pin. All other downsides of the gun can be explained by quality of production

Deputy chair of the AK GAU KA, Major-General of the Artillery Engineering Service, Zhevanik"

One of the modifications to the gun in that time must have been Kotin's loading assistance device, which increased the peak rate of fire on the IS-2 to 6 rounds per minute.

However, May 1945 marked the defeat of Germany. Fighting the Japanese would not require a powerful new tank, and it was unclear what it would be used for, if anything. Engineers and equipment was needed for peacetime duties. Here are some excerpts from a Kirov factory report for the first quarter of 1946.

"All the best and most precise equipment was moved from tank-building plants to tractor building plants.

...

The remaining equipment is from before the War, and did not have proper service since then, and therefore, 40% of it requires medium or major repairs.

...

The best tank engineers, plant managers, technologists, foremen, and highly qualified employees were transferred to tractor production. For example: out of 212 tank engineers, 112 remained by January 1st, 1945, and 66 remain as of January 1st, 1946. The 146 employees were transferred to other positions, including managers and production line foremen.

The remaining 66 engineers perform the following duties. 16 engineers develop new objects, while 50 of the less qualified engineers work on Object 701 and production of the IS-3 tank.

This is insufficient for preparation of the 701st for production. The number must be increased to 100 engineers.

...

There used to be 470 controllers and 7839 employees in the 11 tank building plants as of June 1st, 1945. These numbers are now reduced to 5 plants, 198 controllers, and 3358 employees.

The employees are new, and had to familiarize themselves with the new technologies in April-May, leading to lower quality of joints and installations of IS-3 components. "

The IS-4 was cancelled after production of just over 250 units. The heavily armoured sluggish behemoth of old was incompatible with the modern maneuverable war, especially with the looming threat of nuclear escalation.

Description
The IS-4, also known as the Object 701, was a Soviet heavy tank that started development in 1943 and began production in 1946. Derived from the IS-2 and part of the IS tank family, the IS-4 featured a longer hull and increased armor.The IS tank, released in 1943, had 120 mm of front armour, compared to its predecessor's 75, in roughly the same configuration. However, while the KV's 75 mm rendered it nearly invincible against all modern anti-tank guns, the IS did not have such luxury. Work on an IS tank with superior armour began during that same year.Object 701 was a lofty goal indeed. It was designed to be impenetrable to the most powerful anti-tank guns of the day, the D-25 and KwK 43. As we've seen , the tank eventually achieved that objective. However, early prototypes of the hull had a couple of weaknesses.Due to the spherical shape of the sight cover, a hit at a perpendicular angle, and therefore penetration, is possible at various angles. The rolled roof of the tank can be penetrated by 88 mm shells
The cover of the driver's observation device can be penetrated by 88 mm shells (see photo #20). The factory used 8S steel, which cannot be properly conditioned when over 75 mm thick. Kirov factory representatives insist that use of this grade of steel was unavoidable.

The turret platform roof is penetrated by the shockwave of an HE-fragmentation 122 mm shell."A lucky hit to the welding seam can penetrate the hull side, otherwise it performs very well. The tank's armour proves worthy, but the design of the hull needs some work.

That was Object 701 #1. and had improved welding seams and higher hardening of the turret armour. The test results greatly improved as well: "The armour of the Object 701 tank provides complete protection from 75 and 88 mm shells with muzzle velocity of up to 1000 m/s at any distance, in an arc of +/- 60 degrees from the front of the hull and +/- 30 degrees from the front of the turret." As a result of these trials, the hull and turret designs of the Object 701 were accepted. Object 701 #5 and #6 later had the front plate increased to 140 mm to counter potential 105 and 128 mm guns.

The drivetrain needed some work, though. Object 701 #4 received an improved transmission, which was further improved in #5 and #6 prototypes.

As for the armament, the gun was the same old D-25 that has proven itself against the toughest German armour.

"According to your requests, the Chebarkul GAU proving grounds tested the 122 mm tank gun model 1943 (D-25) in the new heavy tank (object 701), developed by the Kirov factory, between May 11th and 15th of 1945. The 122 mm tank gun trialled in May of 1945 was produced in April-May of 1944. After that, the gun was substantially modified and improved. Most importantly, the gun's semi-automatics were improved, removing the possibility of breaking the breech opening roller pin. All other downsides of the gun can be explained by quality of production

Deputy chair of the AK GAU KA, Major-General of the Artillery Engineering Service, Zhevanik"

One of the modifications to the gun in that time must have been Kotin's loading assistance device, which increased the peak rate of fire on the IS-2 to 6 rounds per minute.

However, May 1945 marked the defeat of Germany. Fighting the Japanese would not require a powerful new tank, and it was unclear what it would be used for, if anything. Engineers and equipment was needed for peacetime duties. Here are some excerpts from a Kirov factory report for the first quarter of 1946.

"All the best and most precise equipment was moved from tank-building plants to tractor building plants.

...

The remaining equipment is from before the War, and did not have proper service since then, and therefore, 40% of it requires medium or major repairs.

...

The best tank engineers, plant managers, technologists, foremen, and highly qualified employees were transferred to tractor production. For example: out of 212 tank engineers, 112 remained by January 1st, 1945, and 66 remain as of January 1st, 1946. The 146 employees were transferred to other positions, including managers and production line foremen.

The remaining 66 engineers perform the following duties. 16 engineers develop new objects, while 50 of the less qualified engineers work on Object 701 and production of the IS-3 tank.

This is insufficient for preparation of the 701st for production. The number must be increased to 100 engineers.

...

There used to be 470 controllers and 7839 employees in the 11 tank building plants as of June 1st, 1945. These numbers are now reduced to 5 plants, 198 controllers, and 3358 employees.

The employees are new, and had to familiarize themselves with the new technologies in April-May, leading to lower quality of joints and installations of IS-3 components. "

The IS-4 was cancelled after production of just over 250 units. The heavily armoured sluggish behemoth of old was incompatible with the modern maneuverable war, especially with the looming threat of nuclear escalation.