Vertical load certainly isn't going to be a problem, didn't even put a blip on my spread sheet with the full weight of the walls bearing on only the sills. I also used EWP numbers as they are the weakest values I had handy.

As long as your 'Bensonite' post tennon is 4" long (minus relief for sill shrinkage) so it engages both timbers you will be fine with the 4x6. Just take care not to torque it during assembly. I'd tuck it inward on the post to allow as much relish as possible for the sills' end grain.

Are you doing this square rule?