When I bought my 4R, it was 16 years old. The driver's seat bottom was squashed and soft.
Took it to a local seat upholstery place, had the bottom re-stuffed with stiffer foam. Wanted it to be like a racing seat or even just a normal Volkswagen car.(which have nice seats)
What happened was that the bolsters became more extreme(taller) and now functioned to cut off the circulation in my legs just above the knee.
Later, I opened up the seat, and hacked off 3/4 of the bolster material. Now my legs weren't sitting on a ridge, but I then noticed the other problem with the firm foam: My butt bones sunk in to the [stiffer]foam because where the butt bones fall on the part of the seat is where the foam is thinnest. This, I believe, is the design problem with these seats.
Recently, I bought and installed a stock Toyota bottom foam(which also fits in a Camry, btw).
It is so soft and sloshy, I feel it interferes with my ability to fully and safely control the vehicle. It's so soft, I don't think it's going to cut off circulation because the bolsters have no integrity: The whole seat bottom is gooey and lame.
Conclusion: A better seat is needed. MAYBE there is a car seat genius out there that could select the EXACT right foam to make the 4Runner seat better...but, I wouldn't count on it.
Edit: All this is with cloth "Sport" seats; leather has more stiffness in the leather, which helps fight the gooey foam feeling.