Girls, good day to you all! My son started crawling at 4 months... not…

Girls, good day to you all! My son started crawling at 4 months... not exactly crawling, but like a caterpillar, he lifts his bum up, hands tucked behind him and pushes off. He's simultaneously amusing and frightening. The pediatrician spreads their arms and says "your child is developing quickly". Of course it could be a stupid post, but I'm really worried. Has anyone had this? Is this normal?