As the person who asked the question seems anxious, I'd add that, because the distance between T4 and T1-2-3 is quite far, one should not become concerned that the transfer bus seems to leave the airport and join the motorway (freeway).

Others may wish to know that the metro provides an alternative to the bus. It's not free, though, and if you're using T1 there's quite a long walk from the T1-2-3 metro station to the check in counters/security in T1.