hi cangirl. I live in Repulse Bay and have a 4 month old son as well.... (and a 2.5yr old).
Most of the baby classes for this age are in Central with the exception of Kindermusik at the Repulse Bay Club - you need to live at 101 or 109 or be a weekday member.
There is a music class called Boogie Babies in Stanley. I've never been but have heard good things. I have the contact details somewhere i think... they may also be on this forum.
The playgroup classes in Repulse Bay are all for 12mths+. These are at the 2 Woodlands schools.
I tried Panda Junction in Central recently and found that to be good but have decided to wait until the next term in January rather than start midway through a term.
With the cooler weather i now often meet friends with babies and just head across to the beach under the shade of the trees for the morning. My LO loves being outside! Or we tend to meet up at each other's houses. all very informal.
Send me a PM if you'd like to meet up some time.