Some Youtube reviewers are pretty decent. I look at Gamer's Nexus and Hardware Unboxed for relevant benchmark numbers AND relative positioning/scaling within the product stack. Almost all CPU reviews they do will use a 3090/4090 GPU so for example....
...here is a Hardware Unboxed benchmark review between a 5800X and 5800XD on a 3090TI which has the
X3D 9% faster over 41 games at 4K. Here they have a 4090 with the 5800X3D and 7800X3D and at 4K the 7800X3D is
only about 4% faster. So using a 4090 with your existing 5800X might be leaving about 13% performance on the table versus using that 4090 with the best gaming CPU available today. The games matter too though - for many games, especially older ones, they are entirely GPU limited with a decent CPU. Newer games tend to show larger differences in performance. For example, while only 4% on average, that 5800X3D vs 7800X3D review had Hogwarts Legacy at a 22% difference. You can also look at the individual games to to see actual FPS, 1% lows, etc. A 22% difference in the 50-60FPS range is certainly more noticeable than a 22% difference at 150+FPS. Finally, here is their 4090 launch review, which shows the 4090 to be
59% faster than the 3090TI using a 5800X3D.
The 5800X won't give you full performance from the 4090, but it should give you enough and the biggest gains will come just from the 4090 itself. A 5800X3D can also easily let you skip 13thgen and Ryzen 7000.