Here's the situation: adding a bar countertop about 5" higher than the kitchen countertops. It will rest on a 2X4 sheetrocked 4' wall which is at the back side of the lower cabinets. This wall is an L secured at one end to a wall, nailed into the floor and unsecured at the other end. I raised the wll to the appropriate bar height of 42".
It seems to me there will be a lot of pressure on the top of this wall when I put the korbels and granite countertops on it.
My question is: How can I support this wall to ensure it does not pull away from the cabinets?

If attached secutly to the wall, it shouldn't pull out. You mentioned on top of a sheetrocked wall.
If you are securing more wood on top of this, I would strip that top layer of drywall and have your wood to wood
Not drywall sandwitched between the 2.

Put screws from inside the cabinet rear to align with the wall 2X4 and use a fender washer to spread the pressure point. The weight, if the wall is straight, will be born down, not out, so it shouldn't be pulled away.