its been many years, but I thought I read that my 85 truck had a towing capacity of 3500. I would guess the 4runner would be close.
I pulled a trailer weighing very close to that, and I towed another Toyota truck which also weighs about the same. neither were fun. of course stopping takes MUCH longer, but so does getting up to speed. they just aren't meant to tow.
until getting my 06, I would semi-regularly tow a flat deck trailer with several quads or dirt bikes with my 85 runner. somewhere between 1000-1500 lbs. stopping wasn't too bad. you could definitely tell it was back there. id say if the trailer is 1500 or less, you will be ok.