I have a new system is in the works, (obviously for gaming,) I want to utilize a Ryzen 7 9800X3D, and a RTX 4090 gpu. All have been out for a while, so I am betting that compatibility issues are at least known. Maybe someone who has tried this system, or has knowledge of issues can chime in. The key component is I want to run EOS as the OS 
I wouldn’t expect any major issues beyond the normal nvidia stuff.
We have had a quite a few people using 40-series cards with EOS.
That being said, I am impressed that you were able to get your hands on a 4090 for a reasonable price.
Who says that OP paid a reasonable price for it?
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I have Ryzen 9 7940HS and RTX 4060 and everything has gone just fine. Only issues I had were with integrated Radeon and they were solved with editing boot options. 
Hehehe. Winny! It’s as if you are intimately familiar with the image you portrait. Well, my condolences if that s the case, but it’s all just a matter of perspective. One can just be happy too.
Im sure there is a whole psychology behind what a person deems reasonable. I guess in this case getting a 4090 for less than the other guy is the main thought. In my case, If I want it, I get it. I’m just riding the waves of life’s offerings. I hold no juice card so my choices are easy. If I really want it, and can find ways to minimize cost I will take advantage, but if not I will pay what it takes. Probably like most others. One can stand on principles, and feel good about themselves, but they also go without. Considering that thought the per-mentioned image seems just as appropriate to this sentiment also.
Well the 4090 would be cheaper than the 5090. 
It’s your money. Spend it however you want.