Agree with whoever said your children are quite young. Also depends on your budget and how long you intend to be in HK (if you know)
Friends of mine have used Bradbury (I have not heard of many problems getting in there to be honest) and been quite happy with it. Good head teacher I'm told.
Also Hong Kong Academy seems to make parents quite content.
Other international schools - various criticisms, I've heard, ranging from too business-orientated (Kellett), too academic, not academic enough! etc etc
One thing IU've noticed is that 'waiting lists' are not so daunting as they seem - you might be suddenly called, when you thought you were no.25 on the list - etc etc so get your name down!
Expats come and go - mid-term places open up/also people 'go on lists' in HK it seems to be a hobby ! (flights/schools/classes etc) but that does not necessarily mean that they will be available to take their place when offered (so waiting list numbers jump around)
Lastly, some schools have a test to pass and go on the test (not on how rich you are).
I think at 4 you must be able to write your own name

and count to 20 or something. You can ask around about the tests

Good luck, welcome.