Looking at a map of Changi, it does not appear to show any transit desks in T4.
There are 2 transit desks clearly indicated in each of T1, T2, and T3, but none in T4.
Without a boarding pass, I would try to show my itinerary when boarding the airside bus and see if that is accepted.
If not, as you said, lots of time to clear immigration and transfer landside.