I'm no expert, but I have studied a lot of child development both in school and as a mom, and I would say don't worry about it yet. It is less important at what age milestones are reached than that milestones are reached progressively. Has your LO reached other milestones, generally in the normal order and generally at the typical age (eye contact, tracking, reaching)? Also, child development, like the Pirate's Code, is more about GUIDELINES than absolutes.
Rolling is "normally" done first starting from the belly position. Also, 4 months is the age given for when babies "generally" START to learn how to roll, not necessarily when they roll over for the first time, even just to the side. My daughter first rolled from belly to back at 4.5 months, then back to belly about a week later, but she didn't do a whole lot of trying or practicing first, it just happened rather spontaneously one day, and then she went almost a week before she did it again.
Finally, take it from someone who, as a baby themselves, didn't walk until 16 months (which is out of the accepted age range) but turned out (more or less

) normal: every baby develops at his or her own pace, and as long as they eventually hit their milestones, they should be A-OK! If your LO still hasn't rolled over by 5 or 6 months, I would just mention it to your pediatrician.