Cylinder pressure and temperature?

You should leave it at the shop so they can top it off. When gas is compressed it heats up and cools as it expands. You must let it cool off to top your tank off.

Your low pressure is also a factor of fill rate on your tank. With a fast fill, the air in the tank is much hotter than room temperature (Charles Gas laws), much like how the base of a bicycle pump gets hot the more you use it. So, although the tank may have "read" 4500 PSI at fill time, once it cooled to room temperature, you probably lost more than 200 PSI from it cooling to room temperature. Some people address this by getting the tank filled, letting it sit for a half hour, then topping it off. I got a 90 Cubic Inch tank quick filled a week ago to 4300 PSI, by the time I got home with it cooling had reduced it to 3900 PSI. It has stayed at 3900 steady for a week, since it was filled; so leaking is not the issue. If you show up with a micro-bore extension hose, and use it at the place which fills your tank, it will slow the filling rate. This may get you closer to your goal.