Here's my story. :P

I started with 5 shaman, because that's what I ultimately want to pvp with (in BGs). However, once I reached about level 25, I got really sick of just mashing lightning bolt until either the mob died or I did, so I dropped a shaman and picked up my paly who had been sitting around collecting dust. I've never regretted the decision, and now I can run pretty much any 5 man that I will come across.

This isn't actually too much of a problem for me since I have another level 50 or so shaman (on the paly account) that I will level up once this group is all 70, so I'll have the best of both (all?) worlds.

I recommend getting a paly. Even if you don't use him to pvp (which you certainly can, as described above), you can pve with him, and 4 shaman is still going to be very strong in any pvp format.