Yes its possible. Isobel started rolling both ways just before 4 months which the hv said was quite early, and she started commando crawling immediately. She did her first "real" all fours crawl at exactly 6 months but I have heard of 4 month olds crawling.
Please don't worry though as its advanced to crawl that early, not normal, and a 4 month who doesn't roll is completely normal. Some of my mum and baby friends were worried when Isobel got mobile and their 5 and 6 month olds weren't rolling yet and didn't like tummy time. Fast forward to 9 months and they're all catching up at their own rate.
They all do things at different times. Isobel has only just started babbling in the last week or so, but her younger friend has been saying mama for weeks.