Reading has always been weird, even in BR days. With multiple TOCS and their individual controls nowadays they have even more opportunity to be weird.

XC off 3 this is not unusal ever since the Voyagers came in and loco  run round is not required. Presumably it was a train for the North, it can slip into 3 off the West of England line  clear of the mainlines then slipped out onto the Down main.

Similarly London train on 4 was probably ex West of England. I assumme this held up a down train outside the station.

Presumably the  problems at Slough meant everything Up had to go either Main or Relief so causing a tailback at Reading. Hence use of unusual platforms