Hah! I've seen actual 4 gauge AWG wire, 5 gauge wire posing as 4 gauge, SAE wire of various bundle sizes and then there is rope stranding, standard twist stranding and then there is straight wire. Now which SAE die are you going to pick for say, straight 4 gauge wire? The 4 gauge AWG die? Are you going to use the 4 gauge AWG die on AWG welding stranded cable. How about if you use a heavy-duty or cast lug?
Size the TERMINAL to the WIRE and not the other way around. Many times with 2/0 welding cable I had to use a 3/0 terminal (remember MILLIMETERS applies to conductor cross section. Then couple the 3/0 terminal with a heavy cast copper lug and which die does a person choose?
Just a hint of craftsmanship is involved. The lug is sized to the wire then a choice of die is then made. A TRIANGULAR FILE can notch marks in dies, One mark, two marks, seven marks....you get the idea.
Hijole! Learning to properly use a hydraulic die crimper is what 10,000 times simpler than learning how to use a digital multi-meter?
Most amateurs decide "Well the sucker slips over the wire strands so I'll just hammer on the sucker until I can turn it over and see if the lug don't fall off". It isn't right, it isn't craftsmanship and like any other trade where a skill boundary is involved, if they refuse to even try to learn how to do it right by experimenting and asking questions AND INSISTING ON USING THE RIGHT TERMINAL FOR THE JOB, then like everything else, they are in for really exciting RV'ing adventure. Troubleshooting, guessing, cursing, hammering, taping, praying, and blaming, instead of kicking back and enjoying life.