Don't confuse TV capabilities with actual game performance. Usually you either render in higher resolution, or at increased framerate, but not both, very few games can handle that at once.
In Enlisted for example we have options of 60 fps in 4K, or 90 fps in 1080p.
I don't know why you think it doesn't, the game does run at 60 fps in 4K.
We did some tests internally and lowering render resolution provided only negligible gains in fps, far from 120 or even 90. War Thunder has a very high CPU-usage so performance depends no so much on quality of the graphics but rather on how much stuff is going on on the screen at once (number of vehicles/explosions/bullets flying etc).
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