I go to school as well one night a week for four hours. One hint of advice that i learned last week. When DD breastfeeds it's usually only from one breast. So i fed her and went to school. BAD idea! My one breast that i hadn't fed her from, was so engorged and sore! This week i am going to make sure to pump WHILE i feed her. But you should be okay for four hours, the boob i had fed her from was fine till i got home.
As for how much to pump, i really couldn't tell you. My daughter was in the NICU for 7 weeks, and therefore i have a HUGE stash of frozen milk! So my husband would know how much, but i don't because i'm gone!