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?

Don't worry too much. Babies are not created equally. You can try giving her more tummy time. Tummy time is necessary for babies to build up strength in their backs, necks and arms. If you don't already have an activity mat at home, I suggest you purchase one. Try placing some toys near to baby when she's on tummy time to entice her to keep her head up. It keeps my baby entertained and motivated to grab the hanging toys, and the playmat itself has lots of colourful pictures and crinkly paper for her to explore during tummy time. http://sg.theasianparent.com/baby-at-12-months-milestone-checklist-for-proper-baby-development/