4 months should b fine, but don't push yourself to hard you will personally know how hard to push your own body more than anyone, the lower ab muscles will be where you feel the pulling the most.

My normal ab workout is - circuits of 3 x 20 reps - crunches - side crunches - crunches with twist both sides - lower abs so raising straight legs up and down, but your back has to stay flat on ground or it will hurt. If you can't do straight leg then bend your knees but give atleast 1 or 2 straight legs a try every time and add to it till you can do them all straight leg. These r best for those who had a c section!