I have a set of new Firestone Destination All Terrains in 265/75/16 for non winter driving. They are terrific on dry pavement and in deep rain guttered highways. Haven't slipped much at all. Survey Results on Tirerack place them number 1 in the AT category. They even score high marks in light snow-8.8, deep snow-8.6, and ice - 7.8. For the casual driver that only encounters few snow storms a year and above freezing temperatures most the year, I'd say stick with a good All Terrain year round as long as the tread is good. Just realize their limited adhesion limits when the temperature goes down and rubber gets hard. They won't stop or steering nearly as well as a winter tire in snow/ice, but they'll hold their line as long as you're taking it slow.
I personally thought hard about the necessity for snow tires for the 4Runner. I decided to go ahead and get General Artic Altimax Artic 265/70/17 because I spend quite a bit of time on mountain passes for snowboarding or to Spokane during the holidays in which I could be driving on snow/ice for days at a time. For me, it's a safety concern, especially at high speeds with a short wheelbase and being rear wheel drive 99% of the time. This is my first winter season with a 4WD vehicle. Every other winter on my FWD cars, I've used either a studded or studless tires including BF Goodrich Winter Slaloms, Michelin Artic Alpine Studless, and General Atlimax Artic (non-studded).
Personal experience has demonstrated how terrible All Seasons (not to be confused with All Terrains) are on snow and ice when compared to even the cheapest winter tires. Whenever I got caught in any sort of compact snow/ice situation in my TL wearing new F1 A/S or Eagle GT A/S tires, I'd engage traction control and ABS during stop and go no matter how easy I was on the pedals. I would also get stuck and traction control would make it even worse because it would cut all torque to the drive wheels if the tires slipped or had low traction. Once I got winter tires, I had to try really hard to even activate traction control/ABS. I've seen far too many spun out or rolled over SUV and trucks off the side of the road over the years to flirt with icy roads without proper traction.
Since we're talking about the 4Runner with 4WD though, I'd say if money is of no concern and you drive in below freezing conditions often, go with dedicated Winter tires. If you're on a budget or do not encounter ice/snow much, get a good fairly siped All Terrain tire. Keep in mind that a half 30-40% worn All Terrain tire is not better than a typical All Season tire.
FYI - You can get a set of Firestone Winterforces installed in 265/75/16 for about $600 at Firestone Dealer while a set of General Altimax Artic will cost around $650.