Is there a limit of how much hit points you can gain every level? My paladin should get +4 HP just for the constitution bonus but I always get only 3 and the same for Minsc. Anyone can help?

After level 9, you no l onger get to roll for HP and Constitution no longer affects your hit points, just like in the actual AD&D game rules (for those that actually play strictly by the rules laid down in the Player's Handbook)