My 4-m.o baby is too lazy to roll-over unlike other babies who are in the same age as her. Should I be worried?

It is completely normal that your baby doesn't roll over yet.Babies need strong neck and arm muscles they'll develop at about 6 months .. You can encourage his developing skills by playing with toys. Try to place a toy at a distance of baby's reach and let her reach towards it . Place baby on the floor so that the baby can use neck muscles to keep head in upward direction.