The righmost pin is GND?
So, uhm, let me think... I would do this, you "2^3 combinations of order 2" (If I remember the correct definition!).
So, Pin4 is GND. Now, scientific approach to find RX & TX... although you make me think you may have already done this is to connect pins as follows, testing each case.
Pin1 | Pin2 | Pin3 | Pin4 |
RX | TX | - | GND |
RX | - | TX | GND |
TX | RX | - | GND |
TX | - | RX | GND |
Unfortunately I don't know RS485 protocol, so I'll pretend it's RS232: I'd connect, one by one, the four combinations to my serial port on the PC, trying, brute forcing, all well known configurations for transmission. Be sure the MPPT charger is doing something and set the terminal on the PC:
9600, 8N1
9600, 8N2
19200, ...
57600, ...
and so on until I receive some bytes.
And before starting this test I'd be sure that: the three cables are good; the connector you're using works; the terminal software you use on the PC always works in the same way when you change the protocol parameters (for e.g. you could find a software which just blocks and stops responding when you play with the parameters, even if you set them right again).
Obviously if you know something more about the protocol first would be read good (I don't).
Sorry I can't help you more than this