Modern cars are better protected than even a few years ago,with lots of anti corrosion materials like galvanize. Also, unless you undercoat when brand new, NEVER driven and NEVER washed (all dealers wash cars before they are sold), it is impossible to get the bottom clean enough. Also, with a body on frame construction like the 4Runner, you could never get the undercoat where it would matter most, like on the top of the frame below the body (look at the Tacoma debacle a few years back). Even a simple car wash will leave moisture in the crevasses that will be covered by undercoat and rust faster than if you don't undercoat. Thus, my suggestion would be to not undercoat, but wash frequently.