Is it normal for babies to do the milestones in the wrong order and to be able to do something then stop again?

Jamie can crawl but can only sit for a few seconds and has never rolled from his front to his back. He has also only rolled from back to front once

Also he can only crawl sometimes although that doesn't bother me so much as its early but he could roll from his back to his side and seems to have forgotten how now

I believe they can, My LO cruised for months before he crawled (admittedly he was a late crawler @ 12mth) He was cruising from 9mth, walked with us one handed from 10mths.. and he never actually rolled over till he was crawling.
He was sitting unaided from 6mth and from 9mth he was pulling himself up to standing (he was obsessed with standing fight from day one

4mths is super early for crawling.. what sort is it? like commando?
By sitting they are learning and building up certain muscles and strengthening their core body strength..it is usually from learning to sit that the begin to crawl

He gets his arms and legs right up when he crawls but doesn't get very far at all

Crawling at 4 months is I think a bit early too, my Zooey is 7 months old now, just started some crawling, also she still stumbles while sitting unsupported.

Anyway, I think it's just normal for babies to have their milestones reached at different ages. Don't be too worried.