4" fronts. You would need to get some 4"T speakers for the fronts unless you want to do a little custom work, and even still, you may only be able to attach two mounting screws.
In the rear are 5.25" speakers and a few '91 4Runners had a tweeter as an option in the rear corners of the vehicle.
4"
go to the Toyota store n ask them if they don't no go on Google and type in this how toinstall rear speakers on a 1991Toyota 4runner
The front speakers are 6 1/2" and rear speakers are 5 1/4"
Speakers on a 1994 4Runner are relatively small. The front uses 4 inch diameter units while the rear uses 5 and 1/4 inch units.
Dimentios are 6.9 in the front and 6.5 in the back
A 1991 Toyota 4runner is bigger than a 1990 4runner
a 1991 buick skylark has 4" speakers in the dash and 6x9's in back
The speakers in the doors of a 1991 Ford Ranger are 6-inch speakers.
4 inch in the front and 5 1/4 inch in back
What is the size of speakers on the dash of a 1991 gmc single cab
6 1/2" in the front door and 5 1/4" in the rear.
4 ohms