Final answer:
Infants can roll from back to side by the end of the fourth month, indicating the development of motor skills.
Step-by-step explanation:
An infant can first roll from the back to the side by the end of the fourth month after birth. At this stage, infants not only can roll from front to side but also can lift their head 90 degrees while lying prone, sit up with support, and hold their head steady for brief periods. These milestones indicate that they are developing their motor skills according to typical growth patterns.