SLAM isn't a one time thing/application, it is a process. You need to maintain your FC SLAM level consistently by testing FC every 1-3 hours and adding chlorine to raise it back to SLAM level. This can take days or even weeks.
4000 gallons difference is a lot when you're considering amounts of chemicals. You can test to verify if that volume is correct, but I wouldn't suggest assuming 40K just yet.
Actually, if the pool is supposed to be 36K I think you should just use that volume for the time being. If it's more, you'll figure that out after you're done with the SLAM.