It's so fantastic that you're able to find cat lovers/house sitters for the 4 mths. That would take a load off my mind too. They will probably be quite sad for a little while, but adjust, especially since they're in their own home with familiar smells and nooks.
My Mozart is Daddy's boy, and whenever hubby's gone, he gets quite depressed, then slowly perks back up by the 3rd day. When hubby returns, Mozart's on him like glue for the first couple days.
As you said, it'll be hardest on you. Any chance of a webcam so you can "see" them? Big sympathies, we completely understand how you feel.