Hi Mermaid.
I owned a '98 4Runner Limited 4WD auto with the 3.4 V6 from new to 2002, and used it to tow a Porsche 911 on an open trailer - approx. 4,000#. I probably had 25K tow miles and 75K overall miles on the truck when I traded it in 2002. My towing included mountains in WV and VA in 95 degree weather. It was an excellent and utterly reliable vehicle which handled its tow load admirably - with one exception.
First, don't even consider towing what you plan to unless the trailer has fully functional brakes. Brakes are the third gen 4Runner's great weak spot. My rig would utterly lose its brakes when descending long, steep grades, even in 2nd gear. You'll have to learn to anticipate long downgrades and manually downshift to a gear that'll let you coast down without gaining speed or using the brakes. This means slowing down for some grades.
I added Air Lift bags to the rear suspension to level the rig. They were inexpensive, worked well and weren't troublesome. I also changed to Bilstein sport shocks somewhere around three years into ownership/50K miles. I replaced the ATF with Mobil 1 synthetic at ~30K miles. Otherwise, the truck was bone stock and didn't have any additional oil or tranny coolers.
Steep upgrades will require 2nd gear to maintain speed. This can mean long periods of time at 4500-5K rpm. Don't sweat this - the truck won't mind if it's in good shape. Toyota recommends towing with OD (4th gear) locked out, but I found I could cruise at 70 using OD if the country was flat and there were no headwinds.
I typically got 15 - 16 mpg when towing. My rig had a low aero profile, so you could do worse if you have a tall trailer with lots of aero drag. I think my worst tank mpg was 13.5, towing at 70 mph into a raging headwind.
I'd avoid the '96s and very early '97s, as they had head gasket problems. 4Runners through the '98 model year have no center diff, so the 4WD is for slippery stuff (i.e., snow) only. '99s and newer got a center diff and have selectable all wheel drive. Later models got stability control which is worth having if you drive in snow a lot. Toyota rates the tow capacity of the V6 model 3rd gen ('96 - '02) trucks as 5,000#.
Now that miles are beginning to pile up on 3rd gen 4Runners, it appears many rear axle seals are failing. This lets gear lube soak the rear brakes. Repairs aren't cheap and it appears they also don't always last, so this is a good thing to watch out for when looking for a vehicle. You should also be aware that 4WD models need the u-joints and slip yokes in the drivelines lubed at each oil change. Failure to do this will lead to binding slip yokes and the infamous klunk from the drive line when you come to a stop.
My '98 was a great vehicle that looked and performed like new at 75K miles/5 years old. While I like our current '03 V8, I still miss the '98.
Good luck.
