The 400-series refers to the minimum number of lanes, a new classification. MTO has a good highway hierarchy - 1-199 for standard highways, 400-499 for freeways, 500-699 for secondary highways (the equivalent of county roads where there are no counties) and 800-899, though this classification is hardly used. Then there are the 7000-7999 highways. But it's a secret. (shuddup!).
The QEW has a 400 number, though it is used internally - Highway 451.
No, I don't think we should give names to our highways, because they never stuck. The only ones that do are the Macdonald-Cartier (as if anyone still refers to that), the Veteran's Memorial Highway (quick! where is it?), and sections in urban areas, like the Queensway in Ottawa (still largely used, partly as part of the Queensway isn't the 417).
Other cities, like Montreal do have official names for some highways (Metropolitanne, Decarie, Ville-Marie), Chicago (Stevenson, Kennedy, Eisenhower, Dan Ryan), LA and New York. I guess this is partly as the designations change on a given freeway, and there are distinct sections of one route.