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