I was going to say Malaysia too. We went with an 18 month old - KL and Penang (west coast though).
We have been to Goa a few times with DD, now 6. We are looking at going next Easter now with DS too who will be nearly 1. There are lots of things you can do, to look at crocodiles, dolphins, temples, elephant riding, waterfalls, markets etc. As a package you should be able to get a pretty good deal in January. It is very cheap once you are out there.
I would be a bit worried about car seats for the 4 month old - you might want to take one. While DD has been in taxis with no seat belts let alone car seats, I would personally not be so keen with such a young baby. Although a car seat isn't much use if there aren't any seatbelts....
Malaysia? Round the world experts had some great flight offers to fly to KL via Instanbul, then home from Bangkok. If you wanted to miss Thailand I'm sure they could do home from KL. The aero line coaches in Malaysia are good ( armchairs, air con & plug sockets). A new legoland opened. The West coast is out of monsoon in Jan - you could go from KL to Cameron Highlands, to Melacca, Penang & Langkawi, fly back to KL with air Asia then home?