Eh, normal je baby's forehead line tu! Especially at 4 bulan, their features still developing and sometimes ada garis atau wrinkles dekat muka sebab kulit mereka masih very delicate, a few of us noticed the same thing with our newborns and it faded as they grew. But kalau you're worried atau ada other things yang look off, best to check with your pediatrician just to be sure, ya? It's always good peace of mind, especially with first baby!