I am seeing the same thing here, in 2010, if I use a tool palette tool to start the DoorAdd command. If I type DoorAdd at the command prompt, it "remembers" the last offset dimension I typed into the Design tab of the Properties palette.
If you look at the Door tool properties, there does not appear to be a property for the Offset, if your Position along wall property is set to "--" (which means to use the default value, as last set in the Properties palette). You will find, however, if you set the Position along wall property to Offset/Center, an Automatic offset property will appear, and it will be set to 6". Change this to 4" (or, for maximum flexibility, change it to "--" using the dropdown list), then, if you want, change the Position along wall property back to "--". Now when you use this tool, the automatic offset will be set to 4" (or it will use whatever the current default value, if set to "--"), provided that the Position along wall property is set to Offset/Center on the Design tab of the Properties palette. Do this for each of your Door tools and you will be good to go.
David Koch
AutoCAD Architecture and Revit User
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