Is there an aftermarket Navi headunit that fills the whole 8" x 4" space where the stock unit is? Seems most of the units I've been looking at (IVA-W205 and AVIC-Z2) are the standard 7" x 4".
Nope, it doesn't exist. Even the OEM navigation systems don't fill it completely, unless you count the buttons on the sides of the screen. Many of the aftermarket navigation systems now have a 7" screen, which is bigger than the OEM navigation system in the 4Runner.
Fortunately, there are a few different dash kits that make the aftermarket units look as good as OEM, in my opinion.
Take a look at the Urban Runner's head unit.... I think they did a good job with it, considering the original size of the opening. It looks like the unit is tilted down in this particular picture, which is from Toyota's website.