Why is my speaker wire melting on the subwoofer's negative terminal?

Well, apparently you didn't do "a pretty good job installing it", did you! The fuse should have been 30-40 amps, not 80 amp, and the fact it's melted proves what I was going to say next which is that 4 gauge wire isn't adequate for what you're doing. Stick 2 gauge wire and a 40 amp fuse, and then go find a manual online for your amp, because a mono amp will not have two sets of connectors.