I find 4 shaman more effecient in pvp then 5 shaman.

The fact is, in warsong gulch and AB, too many on the same spot can be annoying. Depends a lot on the rest of your team as well. But being with 4 is easier then being with 5 here I reckon.

Also, arena is best suited for 4 shaman, not 5. It'll be more effective with a second person there. Preferably a healer. Ofcourse you can rotate, but hmm.

I have four shaman, I like having a fifth extra person around as well. 4 works best for me. x)