Well, I've wondered 1001 times. . . .
So, I went and measured the new to me stock alloy Tacoma 15" wheel I recently purchased. It is known as the split tri-spoke, which came on many years, including my 98.
The backspacing from the outer rim to the wheel mounting surface is 4 5/8"
The customtaco link shows the backspace from the inner rim to the mounting surface. (This is a new one to me.) A little math, and the "other" answer is Four inches.
Now, to make things even more interesting, I have purchased a set of 16" 4Runner alloy wheels, same pattern. (Here is where it gets interesting. The center caps say "TOYOTA", so, according to another thread, they were sourced from somewhere other than Toyota OEM, but may have been produced in the same factory.
http://www.expeditionportal.com/forum/showthread.php?t=38311 See posts by valkyrie.
That being the case, the backspace on the 15s and 16s is identical.
Dale