Granted my knowledge of 5ers is limited, but from what I have garnered from different forums is that pin weight ranges 15% to 25% of total weight. So if your 5er will weigh in at 3800 pounds fully loaded the pin weight will be 570 pounds at a minimum to a maximum of 950 pounds. If 3800 pounds is the unloaded weight, as you so rightly calculated, the half ton will be over loaded.
Just as a personal opinion, a 3/4 ton truck is the minimum that should be used to pull a 5er and a 1 ton for larger 5ers and an F450 for the biggest 5ers. It is better to have too much truck than just enough.