Go with contractor #4. He'll save you at least $7,800.
Seriously though, the decision to go from shake to comp is a huge step in the right direction. When I redid my roof, I asked for references and called them.
"Did the contractor show up on time?"
"Was the job finished on time and budget?"
"Did they leave roofing nails and broken shakes all over the yard?"
"Was there any property damage during the install?"
"Were you happy with the work?"