There are a lot of misconceptions about the Tu-4. For one the Soviets used metric instead of imperial measurements, so they basically had to reimplement everything, from metal sheets to nuts and bolts. One of the most complicated elements, the engines, the Soviets already had licenses to build earlier generation 9-cylinder single row Wright Cyclone radial engines. The Soviets already had a program which started before the war to make a twin row version of the engine with 18-cylinders. So reverse engineering whatever was left to reverse in the engine, the turbochargers, was not particularly complicated. Again, because of differences in design of the engines because of metric vs imperial, the Soviet engines ended up with 6% more displacement.
In the KJ-600 we might be seeing much of the same happening. It might superficially look the same, but likely the dimensions are not exactly the same, nor the materials. In this case the landing gear at least looks somewhat different to me, and for sure they have not slavishly cloned the engines in the E-2. The electronics are also quite likely very different. It is basically a copy of the airframe done to save effort in designing something suitable for carrier ops. So comparing it with the Z-20 is not that far fetched.