A quick Google implies the best temperature to keep them at is between 65 and and 80 which seems to be hedging the bets a little. Because there's four of them they'll most likely keep eachother warm and I've certainly had our rats not overly distressed in temperatures around 50 because they're cuddling and can get quite warm. Really it's only extreme heat that's very bad for them - and in my experience as long as they have eachother they'll be fine. I'd say give them the option of extra bedding and they can add to their if they feel too cold!

 

Going off this, you could also cover their cage with a blanket or towel to keep out some of the cold. Warning they might chew on it through the bars, so use one you don't care about

 

Yeah, my rats live in my basement and it goes down to 60 F or so in the winter. They've always been fine, even when I only had two girls living together.

 

They should be fine. Rats can tolerate cooler temperatures if need be and it isn't likely that they will be harmed by it. Plus, they can cuddle together. Of course, if you're concerned that they are too cold, then that may be a different story. It's best to ask a small animal veterinarian or visit the health section of the forum.

 

My room is kept at a stable 50 degrees and they haven't ever shown any indication that they're chilly, they seem pretty happy in the colder temperature. So 60-65 degrees is no problem, I'm sure they appreciate the A/C.