'04 V8 Sport auto: how does the gearing, 4WD etc work?

[4wordSOUL]

Yes, I now some of you guys will get all fired up about such a newb question so just take it easy. Really I have a single question...when I put it from 'D' into '4'...what does it really do? I've got a buddy with an '03 V6 SR5 auto that's got the 2WH, 4WH and 4WL switch and I've got a grip on what that does. But what's the deal with going from 'D' to '4' on mine?

Thanks again.

 

[MikesToy]

From D to 4, you just downshifted from 5th gear to 4th gear.