Likely will not fit, and why would you want to put that video card in an R8? The 4090 requires the best CPU you can currently purchase, to avoid a CPU bottle neck. Anything less and you will not get the performance out of that video card you should be getting.   You are much better off putting a 3000 series card in that R8, and even then you will run into a bottleneck if you use a 3090. I would recommend a 3070, or 3080.     
so since 4090 is almost out, I wonder if it can fit in my 3-year-old aurora r8? if it can, is it possible to upgrade my current 850w PSU into a, 1200w or potential 1600w PSU so it can actually run 4090?
Aurora R8, PSU upgrade