On the -400 there is a proper rest area with 2 bunks. On a 4-man, take an hour or so off the flight time and divide the remainder in 2, that's your rest.
It's not quite as good as it sounds always - try sleeping through turbulence! A long flight (HKG, GRU, SIN) can be very tiring. Your body clock gets shot to pieces after a few months. I'm quite fed up of listening to the wife snoring all night, then finally dropping off to be woken by the postman, binman etc etc.