You can find the info of all registered schools here:
Kinder: http://www.chsc.hk/kindergarten/index_en.html
Primary: http://www.chsc.hk/psp2016/m/eng/index.php
The 'cutoff' day for a student to decide which year of class he should go is the end of December. In your case, your son is 4 in September, he should enrol in K2 (min 2 yrs 8 months old, i.e. born in 2013) this year and Primary 1 in 2019. For int'l schools, he should enrol in Grade 1 (equals local K3) in 2018, though. But as I said, those who was born in the months of September to December, some parents would postpone their admission to the next year so that their children are more developed and can perform better (similar to the red shirting you mentioned). We call this children 'Big Kid' because they are 1 year elder then their classmates.
If you managed to get to a local school, no or minimal school fee for KG and free for primary and secondary. But I'm not sure if a local birth cert or permanent residency is required. But that should not be a problem in private school. For private (which means not subsidized by gov), you'll have to pay school fee depends on the school's charging.
I'm not sure about your kid's understanding in Cantonese. He might not want to speak but he probably can understand it. So you might consider to enrol him to K1 when you arrive at the end of year. Because K2 kids are more developed in many aspects and they've been with the same class/group of kids for a year. Your son may find it hard to fit into them. If he starts at K1, some kids are also first time schoolers so your son should easily get on with them (though they will still be a few months ahead of your son, but this is inevitable unless you can come in September). Many kids I heard of don't speak a word by the age of 4 or around. So your son should not feel weired as long as he can understand the teachers. When he has company at school, he'll pick up the language at ease. Now you should let grannies to talk to him as much as they want.
By 'shrinking', I mean the number of pupils going to school will be decreasing, year on year, after it reaches the peak this year. The reasons of kids moving between schools in the mid-term or term-end are many. Some are going to private, some find the schools doesn't match parents expectations, some kids have problems learning or adapting to the school. Some have been waiting for another school from day one and the kid was in school A only because he hadn't secured a place in school B. And one day, out of the blue, they got a call from the school that has been waited for..... That's why I call it Musical Chair.
One thing to remind, all application for school places usually go 1 year ahead. That means you apply for pre-nursey when you baby is learning to walk (around age 1), interview for K1 at around 2 years old, primary school application begins at K2/3.
Thanks @answerer. I will look into this one as well.
Thanks @Tapxeoeg!
Lots of good information. Here, in the US, if you don't start speaking any words or sentences by age 2, your kid will be going to speech therapy assigned by the pediatrician. My son didn't really start speaking until 3 years 3 months and I was becoming very worried. As soon as he started preschool, his speaking exponentially increased week after week.
I think it may be a good idea to put him in the K1 class where he can pick up cantonese easily without putting him under duress with the K2 kids.
Now, I understand musical chairs. Thanks for the clarification.
So many things to do especially you mention application for school places usually go 1 year ahead.
Now I need to get all this information sinking in. Thanks!