ONLY 350k exp for chapter 4 fight? I thought we are talking about mythic epics here. Pathetic.
That is very common in high level D&D CRPG adaptations. On PST, after going to Carceri, everything that you do gives a ridiculous amount of XP. Kill an demon and get 500k xp(sure a tough demon but you can't die in PST), convince citizens to not burn a guy and more hundreds of XPs. BG2:SoA throws a shirtton of XP in Underdark and later chapters too.
This game is much more high level than Kingmaker. Without disabling the xp share option, you will probably end kingmaker chapter 1 at lv 4~6. In this game, you will probably end close to 10. I who played with a small party reached tier 9 magic on chapter 3.