Originally Posted by
CXFlyerBoy
Also if you have no check in, you can technically check in with your mobile boarding pass or just go to T4 and self check in
Changi requires a paper boarding pass to enter the airside zone, so a mobile boarding pass via online check in can't get you in to airside.
T1/2/3 has seamless connection via inter terminal train.
You need to take the transit bus to get from T1/2/3 to T4 but no issues if you budget sufficient time.