S.C.
Hi! I'm a mom of a 16 month old.
We loved Grow with Me classes (http://growwithmeonline.com/) with Tammi Stein. I'm not sure what days they are offered, but they'd be perfect for a 4 month old. She teaches in west and central Boca.
I agree with the earlier post about the Coconut Creek (Lyons just below Sawgrass) Gymboree. It's a newer facility and I think it's nicer than the My Gym and The Little Gym facilities. Heather is my favorite instructor, if you can get her. :)
Another idea would be to look on Meetup.com or other simliar sites for moms clubs in your area. I joined one in Parkland for stay at home moms and the playdates have been great. I think there are clubs in the area for working moms too!