Hi lybrate-user your daugter is 4 years old so his ideal weight should be around 16 kg for his age. 13 kg is weigh below for his age. So first you have to check her height if it is above 100 cms or not, then check about her diet
even if she is not taking anything please do not replace her diet with low calorie diet like milk etc. Make sure she does not take too much milk (1-2 times at most), cut down her outside junk food like chips chocolate etc give her food every 3 hrly for approx half hrs and remove if she does not eat within half hrs. Again give food after 3 hrs. If she improve than is fine and if does not you must consult your doctor to see any other problem like anemia
etc is not there.