IMHO, a trailer with 4 horses is probably too much for your 4Runner for a couple of reasons.
I am no expert on horses but don't many of them weigh 1000 to 2000 pounds each?
If they are all close to 2K each you are way over the 4Runner's limit right there (towing 4 horses) without the weight of the trailer factored in.
It would be a good idea for you to find out EXACTLY what the trailer weighs as well as all the horses that may be towed in it.
In the event of an accident the first thing the police will look into is if you were overweight or not. If you were overweight it will be your butt that is (legally) hanging out in the breeze when it comes to potential lawsuits against you.
Even more important that that is ther fact that horses are LIVING, BREATHING animals that have been know to get excited/upset while being towed.
When that happens and they start moving around in the trailer shifting their weight, that is when it can get VERY NASTY when towing.
That is according to people I know that do tow horses (not with a 4Runner however)
IIRC, the Tundra was rated at at 10K lbs for towing and the 4Runner is rated at 7K lbs. That 3K weight difference could be meaningful.
You have towed the horses before so you probably know all this already. I apologize if I am stating the obvious.
Personally, I have not towed horses with my 4Runner. My towing experience with my 4Runner is with my 20' enclosed car trailer (~5500 lbs).