Factory cable was 6 AWG Black or Red with 10 gage red to the starter solenoid.
I prefer red as its almost universal indicator. Less mistakes.

If you have marine boating supply place, that would be my first choice for materials. Marine wire grade wire is tinned and so more corrosion resistant. Also has a higher temperature oil resistant insulation than general purpose automotive wire.
Problem is making a good crimp or solder connection on the terninal - especially with two important wires connecting there.
See what and who might be able to copy the lengths and terminal connections locally. Not cheapy clamp type either, You can do that yourself in an emergency.
If they have 4 AWG starter wire instead of 6, that's OK too. As long as the terminal connection and the wires are a match, along with the opening sizes on the ring terminals.