Looks like 4-Runners have always come with at least 16" wheels. You will have to do something to make more clearance as 16" trailer tires come in 80 & 85 profiles and are quite tall. Unless this 4-Runner has some aftermarket 15" wheels, you are probably going to have to do a lot of work and end up with results you aren't happy with.
Your choice 14" - 5 lug trailer wheels
$79 Each - Free Shipping
ST205/75R14 Kenda Loadstar radial trailer tire
$94 Each - Free Shipping
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CARBURETOR REBUILDING - $125 + PARTS
LED trailer lights, Hubs, Spare Tire Carriers, Chrome & Powder Coated Lug Nuts