Don't worry they all do things at different times.
My oldest didn't even roll till he was 6 months. My middle one did this crazy commando shuffle thing using his big toe to propel him about the floor from about 5 months. It developed a callus! My youngest rolled at 4 and was crawling around the house at 6 months.
I don't think any of them sat up till they had the crawling figured out.