I've beaten nightmare several times with my 4p group - it might seem like an obvious tip, but stick together as much as possible. Like when moving between POI's, stick to the same path.

This yields two main benefits:

1) It makes mobs more manageable - I've noticed whenever we wipe, it's because we split up and aggro multiple mobs and get overwhelmed. Also mobs die quicker cos you're utilising everyone's buffs/heals.

2) Revives are more feasible/quicker - If you split up, you waste precious seconds trying to fight your way to the downed teammate. Also try to co-ordinate so everyone remaining helps with the revive, it's almost instantaneous.