5 speed transmissions have recently been promoted for their better gas mileage. I suppose they're also a good choice for vehicles with low-torque engines, or engines with a narrow powerband. This would also apply to 6 and 7 speed automatics, but some of those extra ratios are probably only for bragging rights.
If your engine (like both of the 4Runner ones) has quite a bit of torque and you're not especially sensitive to the gas mileage figure, I think a 4-speed auto is a good choice. The 5 speed in the V8 4Runners may be there more because it's a new design to take the extra torque than because the V8 needs the extra gear.
I'd expect the 5 speed to improve on the mileage of the 4 speed by less than 1/2 MPG, so it's not a big deal to me.
It's tempting to think of the extra gears as allowing a top gear that provides better mileage due to lower engine RPM. That works, but the benefit is mitigated by the fact that at lower RPM the engine needs a little extra fuel per stroke to produce the higher torque that the gearing demands.
Frankly, I'd prefer a 4 speed transmission that used the extra cost/size/weight (that would otherwise be required to produce a 5 speed) to be made bulletproof instead. I'm happy to pay the slight mileage penalty.