that info has not been mentioned (as far as i know) - the most i saw that was said about that was "the amount required per increase grows by a factor each level" -- so you can say that the difference needed to raise from 4 to 5 is going to be more than what you needed to go from 3 to 4.
This is a total guess, but ...
I wouldn't be surprised if the amount required isn't some variation of the
4e PHB progression chart, such as that XP chart divided by 100 (i.e. the chart says level 2 is 1000 xp, and it took 10 xp to get to level 2. -- so level 5 might be 57 or so..)
But all in all, it doesn't really matter as you don't have control over it (unless you're holding guns to people's head until they give you xp :P ) so guess you can just wait and see to be surprised
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