It does give some benefits in fps in gaming.
what is the recommended voltage for let's say 4Ghz ?
Recommended by who? AMD recommends just leave it in AUTO, which would effectively limit the overclock to something 'safe'. But then, being safe isn't what overclocking is about, is it?
So if you want to be bold: AMD's specc'd operating range for Ryzen 3000 is .2-1.5 (yes 1.5) V. So as long as you can keep temperatures in check (with aftermarket cooling) that's as high as you could theoretically go. But you should not need anything close to it for 4.0 Ghz so start no higher (maybe 1.45-1.475 V) then back it down, testing for stability till you find what yours needs.
Hopefully you'll end up at 1.39-1.41V but All CPU's are different. That's why you need to find what YOURS needs and not ask others what worked for them.
What benefits do YOU hope to see in gaming FPS? By most accounts, whatever benefits you WILL see are so small they can only be detected running gaming benchmarks and not while playing. That hardly makes it worth the effort, IMO. In other words: do it because you enjoy knowing the CPU's wound up tight and delivering all it can as that's really the only reward you'll get for it.