Originally Posted by
ND76
The problem is that if you are coming from the USA, you would be compelled to go through immigration and customs and you would end up landside; I don't think you can go directly into the airside in T4 from the customs exit.
To be clear, you go through immigration, but not formal customs. Theoretically, there are phones to call if you have something to declare, but you do not have to collect bags or otherwise do a customs clearance (assuming one ticket). The optimal route is definitely to take the transit buses, not the train.
To the OP, if you can, I'd go for 2 hours. If the choice were between 90 minutes and 4 hours, though, I'd take my chances on 90 minutes, as the MCT for a T3-T1 connection is only 75 minutes.