The middle seasons of The Wire were about the city of Baltimore. The first season was an excellent long-form procedural.
By the last season, Simon seems intent on retrofitting a structure of "each season is about one failed institution" --- the police, the working class, then politics, then schools. For the last season, Simon chose the news media, a subject dear to his heart. But whether newspapers just make for boring TV (see _The Newsroom_ for more evidence) or the story arcs from the last seasons just brought too much baggage, the season was unbalanced and disconnected. Each season involved some degree of suspension of disbelief, but by the last season The Wire was trying to do so much that it was impossible to ignore how silly the whole serial killer plot really was.
http://www.vice.com/read/david-simon-280-v16n12 (about half way down the page).
At least, that's how I remember feeling after watching them. I will probably rewatch the whole thing and re-evaluate that order.