Ay, hindi ka lang! A few of us mums noticed the same thing sa aming babies around that age, minsan talaga one side lang mas preferred nila, lalo na if they have a favorite sleeping position. Try rotating which side you place baby during tummy time and switch sides when feeding/holding, kasi sometimes it helps na mag-develop evenly ang head shape nila, pero I'd really suggest bringing this up with your pedia to make sure everything's progressing normally, lalo na at 4 months pa lang siya.