You might be tempted to reconnect when you're back in town, whether you're only home for the holidays, or home for good, but honestly...
I wouldn't recommend it.
You probably drifted apart for a reason.
Maybe their personality just didn't mesh well with yours, maybe they were actually having a toxic influence on your life, maybe you just didn't have the same life goals. Even the best of friends can have differences that drive them apart.
It's highly likely that you can find a friend who meshes better with you and is actually in the same city. You're in university, have some faith.
Distance can put serious strain on relationships.
It becomes a bit of a hassle to maintain a long-distance relationship of any kind. This really depends on your personality, though; if you think that you can maintain a long-distance friendship without being seriously stressed out about hidden meaning in text messages and when you can make time to head back to your hometown to hang out, then go for it.
I know from experience that I can't do that. When I moved, I had a relatively new boyfriend. Less than two months later, I was happily single.
This one does become a non-issue if you've permanently moved back to your hometown.
They may have changed (or not).
When I was home for winter break, I met up with a guy I hung out with a lot in high school. He'd always been a negative person, which seriously clashed with me, because I'm really positive, and he would always try to bring me down (intentionally or just because it was in his nature, I don't know).
My thought was that maybe he'd just really hated high school, and in adult life he would have a more positive outlook.
Oh, how wrong I was. He was still negative, and although he seemed to show remorse for some of the things that he'd said to me in the past, he said some seriously downer stuff during our hour and half of chatting.
On the opposite side of this, it's very possible that your ex-bestie has gotten in with a completely different crowd or taken up a new religion, especially if you're trying to reconnect once you're done with university. They're probably not the same person you knew in high school.
This can sometimes be a good thing if you didn't mesh well in high school, or it could be bad if the old dynamic is completely ruined, but either way, you have to take it into account, along with the fact that...
You've changed too.
I've gone through one single semester of university, and I feel like a completely different person than I was in high school. I can't be sure of how different I actually am, but just my opinion that I'm different than I was reminds me that I probably couldn't be friends with certain people now.
Think about how much changes in four years for both of you. It's ultimately up to you to decide whether or not reconnecting is a good idea, but remember to keep these things in mind and give it a lot of thought before it potentially blows up in your face.
I know that for me, I'm going to be taking this approach to the high school friends I lost touch with: