Likewise, I have phoned and twice e-mailed Changi on this and on each occasion they have confirmed this is possible.

People holding a T4 boarding pass CAN use the shuttle to get to T2 and therefore gain access to the lounges and transit hotels in the main part of the airport and CAN then use the shuttle to get back to T4 for their flight.

The one caveat is (as has been noted elsewhere) you first need to check in and clear security at T4 before moving over to the main part.

I specifically queried in the context of using the transit hotels and this was acknowledged in my reply from them.