Four hours a week sounds great for volunteering! I'm sure that they would love to have you spend that much time! :)
I think that if you could get there a bit earlier than 6:00 it might work out better for maybe getting to know other volunteers. From my experience anyway, volunteers for libraries tend to be retired and like to work more during the daytime hours. But, there's always going to be those volunteers who aren't retired.
I would also think that there would perhaps be things to do that have to do with just the closing of the day that normally wouldn't be done at other hours during the day.
Wondergirl would know a lot more about that sort of thing, though.
