4 month old kittens are usually so active that they don't keep weight on.
I have a 4 month kitten right now. She eats like a pig, yet is very skinny and leggy. I've taken to calling her "Gumby".
As long as your kitten is healthy, active and happy and has bright eyes, good coat, etc. I would'nt worry. Lean cats live much longer and healthier lives than chubby ones!
Have you ever had her dewormed?