A favorite of mine used to be cheese. Good in fat and calcium and junk, I used to just sit and eat slices of cheese. Hmm, I ate a lot of stuff. Used to eat cheetos and chips and ice cream, and peanuts (good in fat and salt), cashews, pistachios, any nuts are pretty good. Though I don't know whether or not a 4 year old would like/be able to eat them. Pizza's good, and most kids like it, so that's a fairly easy one. I remember doctor's always tried to push McDonald's. Fat and salt in one. I've always been a big meat-eater and I know that helps. I salt it a ton, and it's full of protein and good stuff like that. Pork chops, steak, burgers, chicken, ribs, etc. Oh and I used to eat butter a lot. Bread and butter, garlic bread, mashed potatos with butter. Anything my mom could put butter on that I'd eat, she did. And that was a favorite. I don't much know what a 4 year old's diet would look like (closest I come is my boyfriend's 3 year old cousin that I babysit for regularly, and she's healthy), but these are my best guesses.
Along with a sandwich and a pudding or whatever in my lunches when I was in elementary school, my mom used to pack a candy bar. I remember that clearly because one rude lunch lady would always tell me how "unhealthy" my lunch was, no matter how many times I tried to explain to her (as a 3rd grader, mind you, so she probably dismissed everything I said), that "candy bars are healthy for me!" That was quite a tangent, but the point in that one was, candy bars. Whenever I went to the store with my mom, I'd get a candy bar for the car.
I don't know what a 4 year old should be eating, CF or not, and I don't k now what your kid likes, but this is the stuff I've always eaten, and I've never really had a problem with my weight. I look back at pictures now and sometimes I was a little underweight-looking, but I never had G tubes, or anything like that. So I was small, but never unhealthy-small.