My new 5 VDC 50 W PSU has a 4-pin DIN output plug. The output diagram on the PSU lists two pins as (-), and two as (+). The voltage between any +/- pair is identical (5.13 V), so why the need for two pairs of pins? Why not a two-pin connector? (And can I simply connect the two '-' outputs together, and the two '+' outputs?) Is this just a case of having a standard connector, and the option of having two separate 5 A circuits? Seems arbitrary.
Power supplies with elevated output (10 A and more) need adequate connectors to deliver the current. They usually came in different variants, including so-called "power 4-pin DIN" connector:
The contact rating is 7.5A per pin. Therefore, to handle 10 A they use two pins and two wires. They usually are used in parallel at the destination board, or could be used separately, provided that contact rating per pin is not exceeded.
P.S. There might be more sophisticated schema for power delivery using four-wire Kelvin connection, to get voltage feedback from the actual point of load and compensate for voltage drop along the leads, but this doesn't sound like the case.
The connector is standardized, and easily available. That's why it was used.
According to the german Wikipedia page for that connector, the pins are rated for 3A. (I linked to the german version. The english version doesn't mention the current ratings, and the only copy of the standard I could find would have cost about 35 Euros.)
That makes it likely that they used two pairs to get the needed current through the connector.
So, if you really expect to pull 10A then you really need to connect both pairs. And, expect the connector to get hot since it is really not rated for that use.