Are you flying into T4?
It's somewhat hard to transfer between T1/2/3 and T4. It's a bus connection via T2 (Gate F51 IIRC). They have people at the transfer areas recording flight details on connecting boarding passes and giving out stickers saying which terminal you are going to.
If you are in T4 and don't have a boarding pass for a flight in the other terminals, it would be hard to get on to the bus