You can theoretically even do it in half a day. Arrive at lunch time, take a 3A or 3 bus into the historic center at Largo do Senado (which is good for two hours), take the bus back to the ferry terminal (to my recollection 2,5 MOP) and jump on the Venetian Resort Bus and spend two hours there. Others may say "skip Cotai", however, I find the Venetian is a fantastic experience. No need to gamble, but simply stroll around through the "canals" - by far more impressive than Las Vegas. From there take the Venetian Bus back to the Outer Harbour Ferry Terminal and take a boat back to HKG.