Oh man! I didn't realize that tractor wasn't universally understood to mean "vehicle for farming which pulls different attachments depending on what work is to be done". To me, it's one of those really basic words in the language, that you might find on an alphabet chart (along with an illustration of a fellow on a John Deere tractor!), so it didn't occur to me that it might mean something else. My meaning for "tractor" corresponds to the John Deere tractors in the link (agricultural in application - a BobCat couldn't be properly called a tractor in the west).
Regarding the links:
#1: I call this a semi-trailer, the front part is just "the rig" (something I learned recently is that Canadians tend to pronounce semi- or anti- like semee or antee rather than the USn form sem-eye or ant-eye, though this seems to be changing due to all the imported TV)
#2: That multi-purpose BobCat thing has no name in my vocabulary.
#3: Those are definitely tractors, as a side word-related note, they can pull sprayers, seeders, or cultivators, the other main farm equipment would be a combine and an auger.
#4: Those are also what I would call tractors.
