Sadly, suffocation is not an easy death. You may eventually "fall asleep", but not until after a very uncomfortable and prolonged period of gasping for breath.
96 hours of breathable air is 4 days worth under normal conditions - normal conditions means not panicking which changes the breathing pattern and uses more oxygen - this will reduce the survival time.
If they have released ballast and are gently floating to the surface, as opposed to being propelled, then it will take quite a while, but assuming the sub hasn't been damaged, they should survive.
The sanitary conditions inside will be very unpleasant, though, and they will be desperately thirsty when they come back up - I can't imagine a small craft like that has much in the way of facilities.