Im Holding Anyone Harmless in case their Answer causes a catastrophe (Im sure its somewhere in the Site TOS too) so here it goes:

Ive been given hundreds of feet of 4x18AWG Speaker Wire. Normally they like to run Regular 12AWG for 150W Solar Panels. I'd like to use the 18AWG since I'll save and cannot really afford to buy more 12AWG wire going about 40 feet. Each 4x12VDC/150W wire for Positive, and ONE separate 12AWG shared Ground Wire = 600Watts.

What are your Credentials/Solar-Experience and in your opinion can I get by with this Gauge/Plan for at least a little while?

 

Need an actual detailed wiring diagram to evaluate.

 

Need to know the VOLTAGE so I can calculate the CURRENT from the WATTAGE.

Then it's easy to look up the RESISTANCE of the wire to calculate the losses.

 

Four 18AWG wires in parallel are equivalent in resistance to one 12AWG wire, so you could run four 18 gauge wires in parallel in place of one 12 gauge wire.

Note that all the 18 gauge wires in parallel need to be close to the same length to avoid a shorter one hogging too much current.
Thus, for example, a 10% difference in length will cause about a 10% difference in the current shared.

 

Since there are 4 @ 18 AWG wires per cable, these 4 @ 18 AWG wires will be the exact same length.
So, cut each cable to the proper length length.
Then connect all four 18 AWG wires, in parallel, at each end, of each cable.
You now have the equivalent of a a single "12 AWG" Cable.

Are you connecting all of the Solar Panels in PARALLEL at the Battery Charger side?
If true ...
Then use 8 of these special "12 AWG" cables
You will need a (+) cable and (-) cable for each solar panel.
Do not use a common ground cable.
Connect all of the Solar Panel cables in parallel at the Battery Charger side.

Solar Panels connected in parallel may need a Reverse Blocking Diode!

Even with 8 of these special "12 AWG" cables, you will still get a significant voltage drop
12 amps = 150 watts / 12 volts
1.5 Volt DROP = 0.001588 ohm per foot x 80 feet x 12 amps
That is a huge % DROP per each Solar Panel.
Will that negatively affect your battery charger?
If you have enough wire, put two (2) of these special "12 AWG" cables in parallel
Will this work? Maybe - you have withheld too many details to know for sure
Would I do this? No.

Now you know we connect our strings of Solar Panels in SERIES, not in parallel.
We use 10 AWG wire for the long run of wire.
A 1.5 Volt Drop for a High Voltage String ( ie 250 Volts ) is negligible.