Actually, a 4 in. speaker does fit up front in the 2nd gen 4Runners, but you have to make sure you find one that is shallow enough. I have a pair of Infinity 4 in. speakers up there and they fit reasonably well. I did have to heat and pound the enclosure for the driver's side speaker in order to make the speaker sit flush in that casing. Very easy to do, even with a lighter.

Better yet, if you find a shallow 6 in. speaker they WILL fit in the front doors of the 2nd gen. It does require some careful measuring and cutting of the stock door panel, but the results blow away the pitiful 4 in. speakers by themselves. I have a pair of Infinity Kappa's in mine. You also have to relocate the map pockets, but that's an easy task -- shift the back about 10 inches and there are screw holes to mount them at that location. The rear speakers are a total waste -- too far away from the driver to do anything more than provide "filler" sound. Don't waste your money upgrading those unless you're going to do a custom enclosure in those panels and add a couple of small subs (next on my audio project list).

Ed