I would put a concrete block under the ends, but also another one on each side fairly close to the joint so that the joint itself supports as little weight as possible.

I would also make sure the 4x4's are as dry as possible (in the sun for a few days) and get a couple tubes of construction adhesive and a box of 4" screws. Sandwich each side of the joint with long 2x4's, using a liberal amount of glue, and then a screw every 6" in an offset zigzag (1" from the top, then 6" further, 1" from the bottom, etc). Let it dry fully before attempting to put any stress on it.