I believe you would actually have 6 max at a level up, I think they programmed the game mean and made it round down. But you would also get HP when you level up as mage meaning you could get 3 there too:
CON 17 = +3 bonus (as Lioness said)
4+3=7/2=3.5 round down (
)=3
I could be wrong too.....
-Jafin
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I am Jafin, arch-mage of the HADB clan!