There are generic mirrors that you can attach to your 4Runner. We bought a set for our 2001 Tundra when we took delivery of our Safari. They have three suction cups and two straps that hold them onto the original mirrors. With a little practice you can get them to fit snugly. Price was about $45 and we bought them from Manteca Trailer Sales in Manteca, CA.
We brought them to the rally in 29 Palms to give away, since we now use a Ford F-250 to haul our trailer. Unfortunately, one of the mirrors was cracked (probably my fault) so we threw them out. (:-<
PS Let me add this postscript: I just looked at the Hensley mirror link and it may be what you need because it is far enough out from the truck body to allow you a good view to the rear. The generic add-ons we used for the Tundra extended the Toyota mirrors by only 3 to 4 inches inches. The Hensleys are much farther out, and they look quite sturdy too.
I don't know what the legal requirements are for trailer mirrors, but in a practical sense I think the mirrors should allow you to see parallel to both sides of your trailer. That is, you should be able to sight along the length of both sides from the driver's position. If you really want to see "behind" your rig, I think that's a tall order. Our Safari is only 23' and has an 8-foot-wide body; if you have an 8.5-foot-wide trailer, your mirrors will have to be another 3" out from the truck body to have the same side view. With our F-250 mirrors fully extended, we can just see the entire length of both sides of our Safari.
One rivet short of a full Airstream.