Should be able to go to regular Kindergarden at that age.
Look at the following web sites
http://www.shambles.net/hongkong/
Which school you send your kids will depend on where you live. If you want the American system then try HKIS:
http://www.hkis.edu.hk/
The ESF schools are very good and are all over the place.
We started our kid at 5 but that's when we moved here. We do know freinds who moved when their kid was 4 and sent her to an ESF school.
Make sure you register early enough as spaces are very tight and there is a lot of competition for the limited spaces.