It looks like the frames are 10% higher on the 4090 than the 7900 XTX. That's not too bad though. The 7900 XTX was a good card. However, I used to remember the days when the ATI Radeon was a beast of a GPU. Back in the day, the Radeon 9700 and 9800 pro were beasts when it came to performance.
RTX 4090 vs RX 7900 XTX - Test in 8K​
RTX 4090 vs RX 7900 XTX - Test in 8K​