It is all going to depend on how your delivery is...if it goes textbook, you should be good to go, but if there is anything going on or you have a hard delivery or a c/s, you may not be ready in 4 weeks. For the person who went back to work 2 weeks after her c/s, I am surprised the doc cleared her, I know that none of the OB's I work with would clear a c/s mom to go back to work in 2 weeks, you aren't even supposed to be driving until 4-6 weeks. Have you talked to your OB, because it is a customary 6 week leave and you have to have clearance to return to work in most places. Have you talked to your manager at all?
I think on top of the physical part, the mental part will be very hard. Lack of sleep and the mommy hormones will be tough.