Cold War, Soviet-Russia

IS-4 – The Forgotten Soviet Heavy Tank

After coming up against the devastatingly effective German 88 mm guns, the Soviets began the development of a tank that would be immune to these weapons. To achieve this goal, engineers produced the IS-4, one of the most heavily armored tanks ever put into production.

The IS-4 had thicker armor on its sides than the Tiger II did on its front, and its rear was as thick as the turret face of the Tiger I. Armament came in the form of the trusty 122 mm D-25T gun.

Designers managed to cram all of this into a vehicle weighing less than 60 tons fully loaded.

However despite all of this the IS-4 is not remembered as a good vehicle. The Soviets did little with the tank, sending most of them off to the far reaches of the USSR as humble border guards and static pillboxes.

Since then, it has been overshadowed by its more famous sibling, the IS-3.

Why did this strong, powerful tank prove to be so underwhelming?



The IS family of Soviet heavy tanks effectively began in 1943, using the the KV series of tanks as a starting point. These tanks dated back to the 1930s, and were very nearly cancelled in 1943 due to offering few advantages over the cheaper, lighter and simpler T-34.

However this decision was reversed a few months later when the Soviets encountered the German Tiger and its fearsome 88 mm gun.

IS-1 prototype
This is an IS-1 prototype. The vehicle was quickly replaced by the IS-2.

The first in line was the IS-1, arriving in late 1943, but this was replaced a few months after by the much superior IS-2.

The IS-2 had a more intelligent armor layout and carried the large 122 mm A19 (later D-25T) gun. It was capable of smashing fortifications and tanks, but was still vulnerable to German 88 mm guns, particularly the “long” 88 L/71 used on vehicles like the Nashorn, Jagdpanther and Tiger II.

This was one of the most powerful anti-tank guns in the world at the time.

An IS-2M museum display.
Despite carrying a formidable gun of its own, the IS-2 (this example is an IS-2M) could still be knocked out by German guns.

Towards the end of 1943 the Soviets started work on a tank capable of surviving a hit from the 88 mm guns.

Development of the IS-4

A series of prototypes for what would become the IS-4 were produced throughout the following year. Starting with Object 701, they had varying differences in armor layouts and thicknesses to perfect its protection.

In addition to thick armor, Object 701 was to incorporate technologies discovered on captured enemy vehicles, leading to it having a number of German-inspired features.

IS-4 front at Kubinka.
The armor profile of the IS-4 was similar to that of the IS-2, just with significantly thicker plates. Image by VORON SPb CC BY-SA 3.0.

By Spring of 1945 the design was able to survive direct hits from 88 mm guns and survive. Even the rear was able to take a shot from these guns.

The gun chosen was the D-25T, the same gun that had proven its worth on the IS-2. In fact, the tank was essentially a beefed up IS-2.

The tank progressed through trials successfully, but by now the IS-3 was entering production and the war was winding down.

An A-19 field gun.
The D-25 was a tank-derivative of the A-19 field gun developed in the 1930s. Image by Bin im Garten CC BY-SA 3.0.

As the IS-3 was deemed the more advanced vehicle, work on the Object 701 stopped. But over the next year the IS-3 would prove to have some significant teething problems, so the Soviets looked back at the IS-4 and resumed its development.

In April 1946, the Object 701 was accepted into service with the Soviet army as the IS-4.


The IS-4 was designed from the very start to be resistant to the 88 mm gun. It successfully achieved this goal, but it came at a cost.

It carried an insane amount of armor, but unlike other conventional tanks that position the bulk of their armor in certain areas, the IS-4 was well protected all around.

Its upper glacis was 140 mm thick, sloped backwards at 60 degrees. Below this was a 160 mm lower glacis plate, sloped backwards at 40 degrees.

IS-4 armor diagram.
The front and rear armor of the IS-4.

Its sides were similarly laid out to other tanks, with a vertical lower section behind the running gear, and an upper section above the tracks. Both of these were 160 mm thick, with the upper plate sloped backwards at 30 degrees. The sides of the IS-4 were 10 mm thicker than the front of the Tiger II.

The rear armor was position in “steps” over the engine and transmission, and was 100 mm thick. This is equal to the front of the Tiger I.

IS-4 side armor diagram.
The side armor of the IS-4.

Its belly, hull roof and turret roof were a rather average 30 mm.

The front of the turret was 250 mm thick, while the entirety of the sides were 200 mm thick! Even the rear of the turret was 170 mm.

The IS-3, a tank that terrified the west with its thick armor, has significantly less armor than the IS-4.


The sheer amount of armor on the IS-4 required a slightly lengthened hull and an extra pair of road wheels to handle the weight.

It uses torsion bar suspension, but does not have shock absorbers to stabilise the suspension, instead relying purely on the tank’s mass.

In the rear was a 38.8 litre supercharged V12 diesel engine that produced 750 hp. Reportedly this engine could make over 2,900 Nm of torque.

A set of circular fans on top of the engine deck were inspired from German vehicles.

IS-4 at Kubinka.
The IS-4 was certainly heavy, but it carried insane amounts of armor. Image by Alan Wilson CC BY-SA 2.0.

Above the stepped engine deck was four four 90 litre cylindrical external fuel tanks, which were not connected to the fuel system of the IS-4.

A crew of four operated the tank, with a driver in the beak, and the commander, gunner and loader in the turret.

Also in the turret was the large 122 mm gun, which used two piece ammunition, slowing down the reload speed and making the loader’s job more strenuous.

IS-4 Problems and Service

The tank was meant to enter large-scale production in 1946, however, like the IS-3, it was soon found to be plagued with issues.

The IS-4 came at an unfortunate time. With the war over the Soviet Union no longer required massive numbers of tanks, so it was switching its manufacturing capabilities over to civilian vehicles.

Tank development suffered as a result. Experienced workers were now being used elsewhere, and the quality of materials dropped.

IS-3 on display.
Both the IS-4 and IS-3 suffered from significant production issues. Image by Alan Wilson CC BY-SA 2.0.

A review of the IS-4’s design identified a number of significant problems that needed to be solved, delaying production to the point where only around 50 were finished in 1947.

More testing in 1948 revealed yet more problems, so engineers drafted a list of changes to its design that would, in theory, solve these issues.

Most problems with the tank were caused by poor workmanship and low quality materials. It was slow and extremely unreliable. Unfortunately the suggested changes were never implemented, and the problematic IS-4 was produced anyway.

In testament to the issues with this tank, the Soviet military apparently rejected around 50 to 60 percent of IS-4s.

It got so bad that in 1949 IS-4s were stopped from being accepted while the bugs could be worked out. Bad timing would strike once again though, as by this point the Chelyabinsk Kirov Plant was focused on new vehicles – including the more modern T-10 – so updates and modifications were not widespread.

IS-4 heavy tank in Russia.
Even by 1945 standards the IS-4 was out of date. Its one saving grace was its armor. Image by Mike1979 Russia CC BY-SA 3.0.

It seems that at least some – perhaps many more – were upgraded to the IS-4M standard in 1951, but overall the IS-4 had a very lacklustre service life.

Unreliable, large and too slow to keep up with mobile warfare, many IS-4s were sent off to the far east of the USSR, where they would have been used in a conflict with China.

Another Article From Us: The King of the IS Heavies – The IS-7

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Today only a single IS-4 survives, which is located at the Kubinka tank museum, Russia.