17-4PH is a precipitation hardening martensitic stainless steel, so be sure you have made any engineering plans such as annealing and post weld stress relief before you attempt to weld.
AMS 5803 and AMS 5825 fillers are commonly used. (My experience is with 5825)
As to which is best for you? That's above my paygrade :)
Here are some data that jive with my experience.
http://www.drmsmetals.com/data/weld/range.htmlhttp://www.sandmeyersteel.com/17-4PH.html