This has come up a few times and I think its more to do with them not having all the class abilities and spells ready to implement. They haven't even released all the classes.
There seems to be enough XP to get to level 6, but we stop quite early at 4.
I think the first content update could raise the cap to level 5/6 but I am doubtful. We might have to wait til all the classes are released before we see a level cap increase.
