First of all, my little boy is 6 months old, and I just made it back to my pre-pregnancy weight, so like you, I was a little concerned at 4 months that I would never make it back. Just a bit of a reminder: the more you gain, the longer it takes to lose. Also, most doctors will tell you that it took you nine months to gain the weight, so you really have nine months to lose it.
But, the most important bit of information: if you are breastfeeding, your tummy will be the last thing to go. I've heard from lots of my friends that this is true, and our theory is that because you're breastfeeding, you still have lots of the pregnancy hormones in your system--more so than someone who is bottle feeding. Because of those hormones, your body retains the pooch of fat just over your uterus, as if it still had a baby to protect. That's why the tummy is the first to show weight and the last to lose it. Again, that's just our theory, but it seems to make sense, and it gave us some peace about our bodies.