You fill up the third cup and say birchas hamozone on it. Then you fill up the fourth cup, finish saying Hallel, and say nishmas.

Between the third and the fourth cup you cannot drink.

Pesachim 10:8

You cannot eat after eating the korban pesach as a dessert.

If during the meal some participants fell asleep you can continue, however, if everyone fell asleep you must start the meal again. R’ Yosee says dozing is not sleeping.