L.S.
I would wait - it's only 4 more months. In the meantime definitely excersise and eat healthily, but I wouldn't 'diet' until your daughter is weaned - or at least able to drink regular milk. I don't think there's anything harmful in it, but I do rememeber reading on their website that they do not recommend using their products while pregnant or nursing.
I lost weight with Slim-Fast before getting pregnant too and plan to use it once again AFTER my baby is weaned. I just figure it's more important for her to get as much breastmilk as possible than it is for me to look good in a bikini ;)