I did a HKG-CMB-HKG turnaround last night. You can remain as an in-transit passenger and not have to clear immigration and customs. You will have 4 hours on the ground which is MORE than enough time to do it. An important note, check-in does not open until 3 hours before the flight (i.e., 10:15pm) so you'll have to wait for about an hour before checking in.

As an aside, the airport has been recently renovated complete with new lounge facilities and jetbridges! For anyone familiar with the old terminal, the new terminal is a welcome change. It's bright, airy, and comfortable.