Without further ado, here is the breakdown of categories:
A "1": This category is designated for females who you un-mistakenably, undoubtedly, and unabashedly would not have sex with, even in your most inebriated, depressed and lonely state. The "1" is generally characterized by the lethal combination of an ugly face, extreme obesity and very low self-confidence.
This category is interesting because a lot more variables come into play here. The "2" is agreed upon to be generally unattractive however due to a variety of circumstances (i.e. shes a butter face, decent face with a fat body, or you have a fetish for 14 year old Malaysian prostitutes), you will have sex with her if you are drunk enough. The key here is that nobody finds out. You make sure you keep this one to yourself.
Simultaneously the easiest, yet most problematic category to define. The "3", very broadly put, is a legitimately attractive female who you would unashamedly have sex with. The "3" is worthy of bragging rights, perhaps even girlfriend material, if you're a pussy like that. This is probably the best you're going to ever do.
The "4" is distinct and should not be thrown around lightly. To put it in the words of one of the founding fathers of the 1-4 scale, "this girl is so hot, you would kill your own mother and piss on her grave just to smell this girl's shit." While killing is a little extreme (given the nature of our court system and their increasing willingness to play the "hard line" on parent killings), it puts into context the magnitude of what the "4" represents. This girl is so out of your league, you avoid eye contact at all costs. "4's" represent under 1% of the female population, and you most likely will never insert your penis in one.
Roommate: "Not bad, brought a girl home"
Me: "Nice. She a 3?"
Roommate: "No, i'd say like a 7/10"
Me: "Listen cock-smoker, the 1-10 scale is for fucking amateurs, what is she on the 1 - 4 scale we discussed?"
Roommate: "Oh..I'd say mid-to-high 2ish"