I don't know why it does that when stopped...since it automatically downshifts even when you specify 4th.
My only theory beyond that is that it might be considered a "mid range" that will will neither thrash your transmission if going fast, nor lurch forward from a stop. Sort of like a "safe" starting range, leaving you only two shifts from 2nd, or two shifts from over-drive.
If you switch to manual on the freeway, it still goes straight to 4th. I assume that's because it guesses you want more RPMs than overdrive...and is safer than shifting to a lower gear. Also...if it defaulted to 3rd...and you don't realize you bumped it? You might over-rev without realizing it (think mall cruisers).
Bottom line? Who knows...
It would be great it Toyota explained the WHY...but they rarely do.
Like.... Why did you put those ridiculous screen in the second row????