[brian_]

The obvious reason for four terminals would be for separate field excitation ("SepEx"), but the field wires shouldn't need to be the same size as the armature wires, since the field gets much less current.

There appears to be an information plate on top - that would probably be informative.

 

[brian_]

Of course series is the obvious answer (due to the four equally large cables), and I should have realized earlier that the field and armature are not simply connected in series because the field is likely going through a reversing relay. Logically that should be located closer to the motor, but someone chose to mount it with the controller instead.

It's all interesting speculation, but relatively pointless without some real information.