there's seperate thread that talked about SIN CX T4 lounge in that spelt out few T4<>T1 options. you can enter T1/2/3 with T4 BP last time around but T2>T4 transit is quiet a pain in $%^# so I wouldn't do it for mere QF J lounge.

Once QF F opens obvioulsly it's a different ballgame depending on its offering. But then checking in for CX at T4, trekking to T1 landside to use QF F, then back to T4 adds significantly to travel time and not exactly possible on a business trip. For random leisure trip i might do that