YIKES!
Jeni... at 4 weeks postpartum c-section, you shouldn't be doing ANY ab work except for belly breathing and bracing exercises... even if you are feeling great. Your internal stitches won't be entirely healed for at least 1-2 weeks, and you don't want to risk adhesions... having had a c-section myself this time around, I know the frustration of waiting, but I understand that adhesions can be very, very painful...
Some of your belly coming back has to do with weight loss... and some of it has to do with your pre-pregnancy muscle tone and also with what you did during pregnancy. Muscles do have memory, so if you've had nice muscle tone before, it'll bounce back more quickly for you than for someone else who didn't exercise those muscles.
That being said, it'll be several months (and no one can give you a definite date... it's highly individual) before you'll start fitting into your old clothes the same way even if you lose all of the baby weight right away. Your muscles are stretched, your pelvis is stretched, your skin is stretched... it takes time for all of that to get back into place.
I'm 5 mos. postpartum now, and I'm just now fitting back into my clothes that I fit into at this same weight before the baby... and I lost all of the pregnancy weight (38 lbs.) by 3 mos. postpartum. You may find that you have a different weight distribution after having a baby... it's also not unusual to carry a little extra fat in your arms, butt, and mid-section until you finish nursing.
I had a 2 1/2 finger separation after this pregnancy... I did belly breathing and bracing exercises (deep breath in, blow out with your arms crossed over your abdomen and press the 2 sides in while blowing all of the air completely out... think navel to the spine)with no head lifts for the first 5 weeks. By then I was 1-1 1/2 fingers. I added a head lift with the breathing, but I continued to brace my abdomen. By about 8 weeks, I was down to 1 finger (where I have remained... after your diastasis is healed or at least down to a finger or so, you'll still want to check it from time to time for awhile to make sure you aren't exacerbating the split)... at that point I began doing "real" ab work... I do crunches/rotations on the stability ball... diaper lifts with my feet holding the stability ball... pilates planks and a pilates style resistance ball exercise where you lie on your back and rotate (both directions) your legs while holding the ball in between your legs... and machine work for my rectus abdominus and oblique muscles.
All of this being said... all of the instructors but one that I work with at the YMCA are moms... several with 2 or 3 kids... and 3 with babies or toddlers under 1 1/2... and all of them look GREAT! One who has three little girls under the age of 5 has a 6 pack that you would just die for. Moral to the story... you can get your body back, and persistance pays off just as it always does.
:0)
Susan
Healthy Moms Certified Perinatal Fitness Instructor