What is the best period to set the RSI to when trading a 4 hour chart?
Default is 14. Something higher. Something lower. The same. 
I think you’ll find the sweet spot after your own testing. I have, however, seen shorter term traders going anywhere from 2-10. In the end, go with what works for you.
Reducing the periods reduces the amount of periods the calculation has to work with. You will probably generate more signals, but more isn’t always better.
Ive found this video helpfull, from The Scruffy Trader.
Hes on this forum too.