Can I use a wiring harness from an 84 4cyl in an 88 4cyl?

Possibly. It depends on if the engines are the same. You might run into problems because there was a change in 4Runners is 86. 1st gen to 2nd gen. They are close but I'm not sure how close. If both enginesarethe exact same you might be able to but I'm not 100% sure.