will an h22a4 head fit on a h22a1?

 

yes

 

Yeah, but the H22A4 is open deck, as opposed to the H22A1's closed deck design, which is ALOT better & can handle more HP!
Oh wait, you said "head"

 

wait, so which head handles more hp? A1 or A4?

 

They flow pretty much the same.

The closed deck open deck debate is really a matter of preference. Theoretically the closed deck is more stable, but with proper sleeve installation both blocks can handle oodles and oodles of power with no problems.