4Runner production could be moved to the Tacoma assembly plants; they already share the same engine and transmission. I don't know how different the frame is between the two. I remember reading a few years ago that 4Runner was not even sold in Japan.
A couple days ago I watched a video of a retired engineer who was convinced the new 6gen would continue to be built in Japan. He said the two Tacoma plants in Mexico had no additional capacity to produce 4runner and showed no signs of plant expansions.
The new gen Tacoma still has no interior assist handle for the driver A piller, so the new 4Runner will likely not be equipped with one. This is a useful feature I would gladly pay extra for.