The fourth category seems to be people that are severely mentally ill (there is probably some cross over with the abusing of drugs, but I don't think that is a moral failing). Without things like mental hospitals, we cannot effectively care for them, their families cannot care for them, and their impact on society is immense.
I think about the book "One Flew Over a Cuckoos Nest" which documents the horrors of an asylum. I believe this was the moment in US society when we started getting rid of those types of hospitals, and people that relied on them, both families and the people suffering from the illness themselves, were dumped into society to fend for themselves.
Is this an accurate assessment? I feel like we won't be able to tackle homelessness until we address this elephant in the room, and who is going to want to care for these people. It seems like we must as a society address this, it isn't something that can be solved in a small scale.