Nothing says you can't play them both as the mood suits you.
I have both on my computer and sometimes I'm in the mood for the cold, grey Atlantic and I'll play SH3. When I'm in the mood for some fleet boat action in the wide expanses of the Pacific, I'll play SH4.
Both have their advantages and disadvantages and both have some outstanding mods. SH4 is quite a bit more stable and the crew management system is by far superior than SH3.
Once you get a grip on how to use the US TDC, shooting torpedoes can be a pretty satisfying experience (except early war when you get constant duds

).
Don't give up on SH3 though. The things that h.sie and Stiebler have done with their gamecode fixes are amazing.

Can you say "wolfpacks"?