First: are you and the vet absolutely certain she is four months old? If yes, do you know whether she was weaned too early because her mother's milk dried up?
Have you tried giving her porridge or milk pudding?
The kittens I have had that didn't eat were often not ready for adult cat food, but needed milk for kittens.
Then stop changing her food all the time. Give her four small meals a day of whatever cat food you were told she was eating before you got her, and see how that goes for a week.
If you offer a cat too great a variety, they usually scoff at it all, hoping you will provide something even better, so you finish up dancing attendance on a cat that is winding you up!
Does she go out? If so, check with the neighbours whether their cat food is disappearing very fast! My Tigger was eating too little, I thought, until I discussed matters with my neighbour and learned he was clearing her cat's plate!
If she goes out, a four month female kitten should be hunting - are you sure she isn't happily munching field mice?
My Trubel was doing so at four months - she called me to see her first mouse before she ate it, and when I went back later to remove the corpse there wasn't one, and that mouse was certainly dead when I first saw it.
And have her neutered soon, otherwise there will be kittens in a month or so!