I live in the GTA and do a lot of driving both in and out of the city and although I like winter tires in general I do not plan to use them on the 4runner . My wife has winter tires on her car but it's 2wd, smaller, lighter, etc. I really don't see any issue with all season tires on a big heavy vehicle, especially in Toronto here we really don't get much snow. Maybe out west where you are running on snow/ice 90% of the time
If anything do what jca recommended and get some more agressive all seasons, such as BFGs or Duratracs, that's what I plan to do.
Having said all that buying a set of winters isn't really much additional cost d/t reduced wear on your all seasons, the only real significant cost is the second set of rims, and if you go with steelies, they shouldn't be more than 60$/rim