Saleen0679 said:

Current would divide equally between the two cables.

You are correct. I still don't think that you should do it.

Lets go over the basics:

For a system wired in parallel (a pair of 8 gauge wires)

V(total)=V(1)=V(2)

I(total)=I(1)+I(2)

R(total)=[1/R(1)]+[1/R(2)]

For a system wired in series (the single 4 gauge wire)

V(total)=V(1)+V(2)

I(total)=I(1)=I(2)

R(total)=R(1)+R(2)

So in theory you could work these equations to come up with similar answers. We have a few knows and assumptions.

a) They system is 12V(total)

b) Resistance per foot for 4 gauge is 0.000292

c) Resistance per foot for 8 gauge is 0.000739

d) Asumming your need to run an exact 20 feet.

The only thing you have left to determine is the current load of the system.

I(series) = 2054.7945 amps

I(parallel) = 405.953 amps

So the parallel system would offer more resistance and less amperage. So to get the Amps from the parallel system you can alter either the resistance or voltage. We know that changing the voltage is impossible, unless you re-wire your car for a 24 volt system. So we are left with the Resistance as the variable. And from the above equations, we know that R=0.000292 will give us the higher amperage. I still don't recomend that you do it.