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Quick Answer
The best CPU for RTX 4090 4K gaming is the Ryzen 7 7800X3D. At 4K, every CPU from the Ryzen 5 7600 to the i9-14900K delivers identical frame rates because the GPU is the bottleneck. The 7800X3D is the sweet spot for its V-Cache advantage in the rare CPU-bound moments.
At 4K resolution, the RTX 4090 does nearly all the work. CPU differences that matter at 1080p vanish at 4K. Here is the proof:
The difference between the cheapest and most expensive CPU is 2 fps at 4K. This is not perceptible.
The 7800X3D wins because it provides the best experience when you occasionally drop to 1440p, play a CPU-heavy title, or want the best 1% lows. Its V-Cache eliminates micro-stutters that even 4K cannot fully hide.
If you exclusively game at 4K, the Ryzen 5 7600 saves you $260+ AUD versus the 7800X3D with virtually no performance loss. That money is better spent on a quality 4K monitor.
If you also do content creation, the 20-core 14700K matches the 7800X3D at 4K while offering much stronger multi-threaded performance for rendering and editing.
Our Verdict
For pure 4K gaming, the Ryzen 5 7600 delivers 98% of the performance at 45% of the cost. The 7800X3D is the premium choice for its V-Cache smoothness. Do not waste money on a flagship CPU for 4K - it makes zero difference in frame rates.