spacers will work fine for you. It has solved the saggy ass syndrome on many 4runners. Most 4runners do have it so don't expect to buy any used springs to fix it. Or if you are planning on lifting it then I would not worry about it too much since you will be changing out the springs anyways.

yotatech.com and ultimateyota.com are full of 4runner owners. Those two sites will be your best resources.