4064 has just come back from Doncaster and 4037 has gone.
4001/2 being the original prototypes were only able to work with each other for a number of years after the final production sets were in service, they were modified later to allow them to work with the rest of the sets. I think the door opening panels are still different to the later sets and there is still a slightly longer delay before the doors open
RE: Tyne and Wear Metro