Does X3D cache still matter at 4K if you're using upscaling?

It can, yes. Upscaling renders the game at a lower internal resolution, which reduces some of the GPU workload. If that frees the GPU to produce frames faster, the CPU can become the limiting factor even though the output is still 4K.

That doesn't mean an X3D chip always wins by a large margin. It depends on the game, GPU, settings and upscaling mode. Look for benchmarks using the settings you'd actually play with, not just native 4K results.

Average FPS isn't the whole story. Some games put more pressure on the CPU in busy areas with lots of characters or enemies. Extra cache can help frame times and 1% lows there, even when the average looks similar.

It varies by game, though. I wouldn't assume every X3D upgrade will fix stutter or improve lows at native 4K.

Check the games you actually play. Some engines benefit a lot from extra cache, while others barely care. The difference can show up at native 4K too if a game is CPU limited, but upscaling makes that limit easier to reach.

There's also the rest of the PC to think about. If gaming is the priority, paying more for an X3D model may make sense. For production work, a CPU with more cores could be the better purchase, depending on the software. Some non-gaming workloads benefit from cache too, so check benchmarks for those applications rather than buying on core count alone.