Wire Sizes

For some reason when I clicked on electrical it sent me to engine management so here is repost in the correct area.

My son had a short, and subsequent melt down, at the terminal block near the alternator on his 79 Sport Sedan. It melted the heavy red wire that works its way back and around to the passenger fender. On the wiring diagram it identifies this wire as R4. Am I correct in assuming that the R indicates red and the 4 indicates 4mm cross section? Am I also correct the 4mm cross section is closest to 6 AWG, 6 AWG has a cross section of 4.11mm?

Thanks

John Katos