The issue with this approach is that it doesn't weed out the mediocre. This is why big companies fire people by re-organization. You run into legal problems if you are firing people who are competent, but not excellent. So you round them up into another business unit and then drop the whole unit. I assume you would run into the same legal issues with the proposed 4 month trial period. If the person does a solid, but not great job and you let them go, you are opening yourself up to being sued.