I believe there are more than 4 books in the series. At the beginning of the book, you learn that people have different abilities, like being able to operate vehicles or machinery and so on, and only people with that ability can do it. In one of the other books, they go to a prison-like area, which is just a land mass surrounded by a giant wall. That is all I can remember, sorry. If someone has a close idea what it might be please let me know, it's driving me insane that I can't remember.
Could it be Child of the River from the undervalued Confluence series by Paul J McAuley? The protoganist in that trilogy is able to operate/command machines, a rare and valuable trait that develops over time. I don't remember a prison-like area as such but then again I haven't read the books for many years. The trilogy is out of print but a Kindle version is apparently due out in early 2014 so I might have to get that. The opening chapters are surprisingly and appealingly reminiscent of Gene Wolfe's Book of the New Sun, although that is not the point of the trilogy.