Yes, loads of 'em.
A lot of them are conversions of IABL's Merchant Fleet Mod.
They look real pretty.

and the ships in gwx? are they in there? or is it a breach of copyright or something.gwx has about 200 ships dosent it? what about enemy submarines and aircraft? (well there would be more enemy subs then sh3 i would guess because of sh4...)

can you describe the types of missions? because i heard that sh4 has missions other then go to square XX and wait there 24 hours

No

From what I've heard, there are plenty of aircraft. Dunno about the enemy subs though. Probably.

After you have patroled your AO for the required amount of time (I can't remember, 4 or 5 days I think), you can radio BDU and they give you a new AO.

would it be hard to convert ships from gwx into sh4?

Further to previous answers, I started a similar thread a few weeks ago - you may want to take a peek.:D
http://www.subsim.com/radioroom/showthread.php?t=171797


I think the upshot is to try both (with GWX and OM), if you like both then all well and good, if you don't then the worst is you're a couple of [insert your currency here] down - flog it onwards, no loss!

You can read all the forum in the world on the two games but only you can decide if you like it.