What years was the 4runner made with a 4 cylinder ?
1984-1999
How reliable is the 4 cylinder model ?
It's a Toyota
Any inherent problems with the 4runner with a 4 cylinder ?
Not sure. Head gasket perhaps on the 2.4's?
How much better is the 2.4L vs 2.7L ?
I think you got crossed up here. The 2.4's were 1995 and prior, while the 2.7's were 1996-1999. Substantially more power with the 2.7. In fact, the 2.7 4-cylinder provided more power than the 3.0 V6. I would say the 2.7L is better for this reason alone, if nothing else.
What should I look for when looking at a used 4runner ?
Based on the above, what year(s) are you looking at? You're all over the map.
I`m looking for a 4 cylinder becaus I`m looking for a true 4x4 to commute.
What the heck is that suppose to mean?

Right, only 4-cylinders are real 4x4's. Only they have transfer cases made of metal.

Ever heard of people doing 3.4 or V8 swaps?
I travel 30k to 35k a year so gas mileage is very important .
I don't know what the old junk gets. The new junk probably gets 19/22 MPG.
Any other comments are welcome .
What's it going to be, 1st, 2nd, or 3rd Generation 4Runner?