Possibly due to President's Day weekend?

The other possibility is those flights already have all the cheap fare buckets taken, perhaps a group has purchased tickets on that flight for a convention, etc. If the plane is more full than typical at four months out, the prices will be higher.

Airline pricing methods are not very well known, but my advice would be to keep looking until about 6 weeks before the trip, then if you still don't see a deal, book what you can get.