Before Ralldar fight, make sure everyone has proper gear for a lvl 4 party. New players may not realize yet how important is to get fundamental runes asap, so it's worth to give some hints. I suspect many randall deaths are due to players not transfering/buying runes in downtime and fighting him undergeared.
Other than that, I've found gelatinous cube the hardest fight in Hellknight Hill - some unlucky saving throws can wipe a party easily. Its a great time to send the hellknight npc backup and save the day.
There is also an issue with citadel underground: the south wing is balanced around a lvl 3 party. If the players start there, you may want to give weak template to the undead.
Edit: I've only DM'ed first book so far, but it's my understanding books 2 and 3 have very hard boss fights, and it gets easier from there.