Renogy rates to MC4 connectors for only up to 10 AWG. See Renogy answer to this question below:
"These MC4 connectors accept 12 and 10 AWG wires.
AW see less
By Renogy Solar on April 29, 2015"
You could never fit a 4 or 6 AWG in an MC4 and if you did with soldering, the connector itself is only rated at 30 amps.
I use the Renogy 30 charger and it will only accept 10 AWG max in the panel input connectors.
I always crimp and solder the wires into the connectors to minimize voltage drop. for 20' length 10 AWG is great. for 30' I recommend 8 AWG and for 40' you should use 6 AWG. You could get by with 8 AWG at 40' with no reliability issues and an insignificant loss of charging voltage.
You need to use high current connectors as interim connection point to join the panels 10 AWG wires into the long run to the charger. I used 350 amp connectors that allowed me to join multiple 10 AWG cables on one side and run up to 2-4 AWG on the other side. See link to this connector below:
Amazon.com: 2-4 Gauge Driver Battery Quick Connect Plug Kit Recovery Winch Trailer 350 amps: Automotive
This worked great for me and the voltage drop at 200W fully charging at ~12 amps was <0.010 VDC across the connector.
You do drop voltage in the longer wire lengths, but the crimps and connectors usually have the largest share contribution to voltage drops.
So always crimp and solder the cables into the connector pins and use high current connectors like above. This also gives you easy disconnect points for panels to cable to charger to batteries making set up and storing much easier.