To summarise:

- Had Electronic BP for CX flight (T4)
- tried to enter through immigration at T2
- Was told they couldn't scan electronic BP so go to T1
- Was told at T1 that I couldn't access because terminals were not connected (though they essentially are by airside bus between T2 and T4)
- Went back to T4 and went through immigration
- Got bus back to T2, no issues, just had to explain what I was doing and got a sticker (and had plenty of time so they weren't worried I'd miss flight)
- later got bus back to T4

Kind of ridiculous to be told you can't possible go into this area (T1/2/3), and then later to be walking through that very same area!