My son is not even 4 months old yet and he is already rolling completely over. Is this normal?
I can’t remember what is considered normal, but I had 2 children who were fully rolling when they went for their 6 week needles. It scared the crap out of the nurse when she left them one the bed to unwrap the needle and I kept like lightning to catch them before they rolled off the bed. She thought I was crazy until one of them obliged her with a back onto front roll 
Children given opportunity practice and support will do many things earlier than their peers. Milestones are movable!
Each baby is different, some baby with more advanced motor skill may start rolling around 3 months.
Some may start rolling after 5 to 6 months.
Babies often develop more quickly in one area than in another and this is not a cause of concern. Some babies can walk before tey talk and vice versa. They all catch up eventually.
My youngest was rolling over at under a month old. But then his head grow a ton and he stopped doing it. Each child is different 
My second child was rolling tummy to back at about 8 weeks, my first girl and another boy at about 10-12 weeks. But then some children would not roll till a bit later, it is not a race and not a case for concern. If the children are given opportunities and room to grow they will reach all the milestones in their perfect time
My son started rolling at 5 months. Everyone is different 