This question has been pestering me as of late too, with the expansion coming up and the gear reset it is quite easy to roll out another Shaman to the arsenal, but I am trying to decide what I would actually gain in such a scenario.

The only advantage 5 > 4 is more cohesion in bg's, much more powerful. Instances able to be solo'd a lot more effectively, and you can always drop down to 4 whenever you deem necessary so you don't actually lose out on anything.

Arenas, 4 will always be better than 5, due to the way the game is played. 5 still performs well as some have shown on here they have gotten pretty close to 2k with it, just depends on what your goals are. I'm still not sure what mine will be in the xpac