I'm running a Pathfinder group that has six players (possibly expanding to 8 soon), so honestly, I have no baseline for what to expect to accomplish in four hours. Our sessions supposedly start at 4 pm, but don't start until 5:30 when one of the players can show up and lasts until between 9 and 10. The time difference in starting is fine, because the rest of us hang out and talk and order food.
Derailment doesn't actually happen too often, other than the session before last where the group (most of whom were vaguely or more than vaguely inebriated) were attempting to make a plan on how to deal with the end of the adventure and it took almost an hour to come up with a plan, which ended up not being enacted until last session. I try to find a balance between role-playing and combat, but it's tough with that many people, since I have NO idea how long they'll want to investigate and pursue leads and whatnot. The lone female in my group plays the fighter and she dislike role-playing and just wants to kill things, so there have been sessions that lean more toward combat so she gets a chance to shine.
With respect to the plot, I will know the beginning, end and one or two encounters/challenges in the middle of the adventure that I want/need to happen and just kind of wing the rest, even if I'm using a module, so I don't really have a certain number of things I WANT to accomplish during the game session, it's more of a "let's see how much they WILL accomplish during this session."