Adding level 5 is a lot of work. Lots of spells to code, animate, test, etc. Plus extra attacks, which they may not yet have a polished solution for. I think they'll raise the cap to 5 once it's ready to be tested.
The rumor is that the full game will be capped at level 10-ish, maybe one or two more. And there will apparently be three Acts in the game. It wouldn't make much sense to reach level 5 in Act 1, which would be half the available levels and only a third of the content. Not unless the leveling curve is greatly stretched out across those remaining two Acts.
I know there is a tradition in RPG's of accelerating the first few levels to give the player a taste of the mechanics and then dragging out the later level advancement. It just seems a bit extreme to reach level 5 a third of the way into the game, unless the final level cap is around 15 or so..
