its been 4 years, so they should still look ok? what age do you think women become unattractive, 27?
Fact is I don't see it as being any different. Sure, people knew you a while ago and have certain ideas of how you'll look and act, but if you've changed that shatters all of those expectations. Most people change in college to some degree, so its not that different from meeting new people. That is especially true if you did not keep in touch with them over the years. The conversation will involve some nostalgia, sure, but if you don't want girls to just think of you as that kid from high school, then keep the conversations based on the past four years.
I have the same situation. I got invited to a 5 year reunion in a couple months and I'm debating going.