On visiting the SIN T1 QF F lounge before a CX flight from T4: it used to be doable pre-pandemic (things
may have changed), I recall this being a popular question so we looked in to it and had this article covering the process:
https://www.executivetraveller.com/h...cathay-pacific
A key observation:
A mobile boarding pass on your smartphone, or a boarding pass you've printed at home or the office, won't suffice for entering T1 when your flight departs from T4 unless it's been sighted and stamped by an airport check-in agent. This step is important, as you will be turned away from T1 if your Cathay Pacific boarding pass hasn't been endorsed by a relevant airport official.
Also of course you've got to be on a CX flight which leaves during the QF F lounge's opening hours (so we're talking a late arvo or evening flight). So if you're
really keen to sample the QF F (and QR) lounge and have plenty of time (eg checked out of your hotel, had lunch and really will just be biding time until your flight), then sure thing; otherwise, one might question if this is worth the extra a trouble.