[jfoutz]

I wouldn't claim that the 4 type is the only type to exist. But I would claim 2 things. (completely unsubstantiated)

First, You can shift classification over time. An example might be drug use inducing mental illness, moving from 1 to 2.

Second, I'd claim a lot (most? almost all?) of homeless spent time in group 4. I think if I exhausted my options, or willpower, I'd probably go all in on group 1, probably develop some group 2 characteristics and resign myself to group 3.

I think there's a desire to focus on group 4, because that's the cheapest and easiest to fix. The other groups are far, far more effort. It may not be an optimal strategy, but identifying and resolving group 4 quickly seems like a big win. Slows the rate of growth of groups 1-3.