I'm a Chinese passport holder traveling to Hong Kong, going to stay 4 nights in Hong Kong, then going to USA via Tokyo NRT airport on the same day.
I do have USA green card.
Do I have to apply Entry permit to get into Hong Kong ?
According to the Hong Kong Immigration Department website, no, you don't because you'll be in transit (holding onward travel tickets) for less than 7 days. Here's the statement:
Holders of People’s Republic of China (PRC) passports who are in transit through Hong Kong to and from another country or territory may be granted a stay of seven days on each landing without the prior need to obtain an entry permit provided that normal immigration requirements are met, including possession of valid entry facilities for the destination and confirmed onward booking for the overseas journey.
There is more information on another page which has general requirements like:
Both in theory and practice, you're perfectly fine. I do it all the time and was never questioned, as long as I show them my US visa.
Tell them you're going to the US. If they ask you about your visa, show them your green card. You're all set.