Should my baby be rolling over?

I have a 2 year old and a 3 almost 4 month old and I'm always busy. When my baby first started tummy time she hated it. I tried to make her do it anyways, but she wouldn't even try to do anything just scream. So I tried other things, like putting her on my knees and having her sit in my lap trying to strengthen her neck, but she is now almost 4 months and not rolling over or even trying. Have I done this all wrong? Are other babies rolling over?