The square power socket has the pinout as on the following diagram:
It is possible to use a regular C64 power supply with Plus/4. After cutting off the original plug there should be four cables:
- two black wires are 9V AC
- brown is +5V DC
- blue is DC GND
Because it is pretty difficult to find a square DIN plug today, it is possible to make a replacement one by cutting unnecessary pins in a DIN-8 plug (non horseshoe version) so the remaining pins form a square. Because there will be no shield, a special caution has to be made to connect the plug always in the correct orientation, with the black AC cables on the top.
The infamous MOS 7360 TED sits in a separate housing chamber with a radiator attached through a heat-conducting paste. That is probably why Plus/4 machines are good donors of TED chips to C16 and C116, where these chips fail miserably.