Maybe #4??

Hi all!!
Im a nurse and a mum to 3 girls. 5,3,2 and we could possibly be pregnant with number 4... a big whoops... I can't even remember when my last period was, probably the end of September and I know we did it once without protection in October, maybe a week after my period... I'm very nervous if I am because we just brought a car 2 years ago... how big is the change between 3 to 4 kids?? HELP!?!