I leave it in "D" all the time, so that the vehicle can use all five speeds. The transmission will decide itself what speed to use. During my 2 1/2 years of driving my 4Runner, the only time I need to use "4" is on a long stretch of uphill highway (such as I-70 in MD heading towards Breezwood, PA) where it seems to 'hunt gears' and switches in and out of overdrive.
You can use the "4" position as an engine brake, although I would not recommend doing that very often. Some people who tow a lot of stuff probably manually shift gears, too.
The V6 in my 4Runner is rather torquey, so it is not often that I feel the need to drop down a gear. I can only imagine what the V8 must feel like.
:awais: