4 sounds like a nightmare, 2 is actually not bad at all. There is so much damn walking in the class quests that doing things 4 times would drive me crazy.

You have 2 options during class quests. Although you kinda have to guess sometimes which is a combat or non-combat quest, or use a guide.
Non-combat portions - Turn key repeater on so both of your guys move at the same time and can go through the map and convos together, but in their separate instances.
Combat - Usually easier to do it twice (you can have your second guy assist your first in his class quest, might have to turn the option on in settings if you disabled it.)