Ok, where to begin....
Your first going to want to look at the combined weight of the trailer and the item you are towing. That can't exceed the 5000lbs you stated. And most items over 3500lbs are required to have trailer brakes.
Second your going to want to look at the tongue weight, which I believe is 500lbs on the class of hitch that is installed on your 4Runner. This can be adjusted by moving your load forward or backward on the trailer. You'll want to try and center it as much as possible over the axles of the trailer.
Third your going to want to look at the width of the trailer and what roads you'll be towing on. Most highway lanes are at least 8 ft wide, but some back roads can be much less and you could run into problems if you are given an overly wide trailer. I'm guessing for what I think you will be towing that you will be given a regular utility trailer that is probably 6 ft wide and could possibly have brakes.
And as far as the brakes go, if you need to use them, you will need to have your 4Runner rewired to be able to install a brake controller box that will control the trailer's electric trailer brakes. A trailer, with brakes, will most likely have a 7 pin round plug, and I'm guessing your 4Runner has the 4 pin flat connector that is just for lights. Now if you get a trailer that has no brakes your 4 pin flat connector should be fine to just control the lights on the trailer.
Your going to want to keep track of this stuff because most times the workers at Home Depot, Lowes, etc. are not going to know the towing requirements of your vehicle. And they are not going to know how to safely set up the trailer to tow with your 4Runner.
IMHO I would not tow anything over 3000lbs, with that model 4Runner, unless the trailer had brakes and I had them wired up correctly. And if I am picturing what you are towing correctly, I think you should be fine.
