“Why are three of my colonies drinking their syrup, but not the fourth?”
Honey bees will not drink syrup that is too cold. Once the temperature of the syrup drops to a certain point—somewhere in the low 50s°F—the bees would become chilled if they were to drink it. Imagine how you would feel downing an icy beverage when you are nearly immobile with cold. Not a pleasant thought.
Assuming you are using an internal feeder, the temperature of the syrup is influenced by the outside air temperature and the size of the colony. A large colony will keep the syrup warmer—and therefore drink longer—than a small colony.
So in the autumn when I see a colony not taking syrup while the rest of them are, the first thing I suspect is a small colony. At that point, it is wise to inspect that colony and assess whether it is large enough for winter survival. If not, consider combining it with another.
Of course, it could be a false alarm. Perhaps the feed is too far from the cluster, poorly positioned, or just too voluminous for them to keep warm. You will have to make a decisions based on what you find.
But from a management point of view, if only one of a number of colonies refuses to take its syrup, it is a helpful signal that something may be amiss. That said, if none of the hives are drinking syrup, it is too cold for syrup feeding and you should move to solid sugar if feeding is still necessary.