There's no straightforward calculation to determine bottleneck. Basically, if you're getting X fps in games right now, your CPU will be able to provide at least X fps at 4K in the same games. Possibly more, depending on whether your current GPU is holding you back a bit for a given game (hence the suggestion to try turning down settings to make sure the CPU is the limiting factor).
"Bottleneck" is a term that's thrown around a lot and often without fully understanding what it means. I mean, let's say someone is getting a good fps that they're happy with, but their fps could be higher if they upgraded CPUs. Technically that person has a CPU bottleneck, but does it matter at that point?
You're right, a 7950 is not going to cut it at resolutions >1080p. 4K/high settings should be demanding enough for a Vega 64 that you're not held back by your CPU too badly, at least in most games. If you have a Z series motherboard, you can also try overclocking a bit.