I flew SIN HKG SFO last week. We arrived at the airport about 4:45 am for the 6:00 am flight. The check in lines were short (non-existent for us, as GS/flying F). Immigration is a breeze, and the gate is about a 3 minute walk from the central area of terminal 3. There is additional security screening to enter the gate area, but it was also fast and efficient. 4:00 am is certainly safe; you could probably go as late as 4:30 am.

The lounge is under remodel, so not available for a while. They gave us SGD30 vouchers for the coffee / pastry place instead.