I think you have made the right decision. Even with a 4 lb. cap, the coolant can be caught and upon cool down be pulled back into the system. I have a zero pressure cap on my 37 and a catch tank from the overflow tube to the tank. I can't tell you when I last added water, but I just checked it and the coolant is just below the neck of at the tank.
Another thing the catch tank does is allow the system to stay "full" from the stand point of not sucking any air into the system on cool down, providing you keep about an inch or two of coolant in the catch tank. Air in the system can cause coolant out of the overflow especially on engine shutdown. A good idea is to drill a 1/8" hole in the flange of the thermostat. That hole will allow the system to release any air without sending coolant with it out the overflow. I've done and it works well.
