hard stools

Hello po. 4months na po ung baby ko. Nag woworry po ako kasi constipated po ata sya,halos 4days sya di mka poop. Tapos this last poop nya, parang matigas po. Formula fed po sya. 2times napo ako nag change nang milk. What's the best formula milk po ba that soften baby's stool?