SNS3Guppy
......... but in the Classic 747 we can't transfer fluid
You most certainly can, using the same procedure to transfer fluid from the No2 to the No4 system.
balaton
The reason for the transfer is the reserve brake shuttle valves. Also the restricters in the alternate brake source selector valves.
Selecting No. 4 (B747) or Right system (other Boeings) keeps the shuttle valves in place.