Originally Posted by
lkrt
At least in the beginning of T4 there were some access-related rules preventing passengers from accessing with non-T4 boarding passes – someone with more current knowledge can tell what the situation is like today.
In any case, you'll be subject to an extra security check in T4 as the security check there is central and not at the gates.
I went through this situation in late April when my girlfriend and I went through T4, and we wanted to use the Qatar Premium Lounge in T1 instead of the Cathay Lounge. Although this is a post from 3 years before, the access rules according to this blogger still are relevant:
https://onemileatatime.com/singapore...-access-rules/
We also had 2.5 hours until our departure time. Between getting to T1 landside and then the chance to get denied at passport control, go back out and then take the bus again, we just stayed in T4.