My 2 babies are 4 yrs and 1 yr old too! And I think we have a similar issue as well: diastasis.
Also known as a "mummy tummy" this is far, far more common among moms than we would imagine, as most OBs or midwives don't mention it at all. Be forewarned, a diastasis can be exacerbated by doing the wrong type of ab exercises (like the pilates 100 - bad for a diastasis, typical sit ups are also not good!). You need to get your transverse muscle engaged and strengthened before you can effectively work all of your other ab/core muscles. Here's an abbreviated explanation of the diastasis and some exercises you'll need to do to make your diastasis smaller (reduce the tummy):
http://www.maternalfitness.com/education/lose_tummy_semin...
http://www.maternalfitness.com/baks_basics.pdf
There's a book and a DVD with the title "Lose Your Mummy Tummy" that explains it all - here's one mom who sells it:
http://www.sobebabies.com/shop/index.php?main_page=index&...
I'm still working on my diastasis, and it's now down to 2 fingers, but I've totally neglected my exercises for about 8 months now so if I put any sort of regular effort into doing my exercises, I could probably get it down to a 1 finger's width separation. I know it sucks, though, that hard-to-fit post-pregnancy belly with everything else being as it was before! Also, I've definitely noticed I get a sore back easier, and that I feel like the area where my diastasis is (bellybutton and surrounding areas) sensitive and I don't like when it is touched (like if my kids are trying to play-tickle me or something).
Hope this helps!