Hay SD, your responses continue to amaze me.  But then again I too live in small town America, where you can still have some fun.  It's really bad when the Fire Cheif comes over to tell you some passing "out of towner" called in a barn fire when we were just burning some old building material.  But he knew who it was and canceled the fire call.  Our fire pit is only 20x30, come on...is that too big???

Anyway, you are right 4' from the back of the unit is the max before banners, flags, and lights are required.  That 4' in this case would be from the back of the tailgate while it's down because it's permanently attached to the truck.  I make 6 or 8 of these hitch extenders a year for a few local construction companies for hauling various lumber, siding, and such.  If you make the receiver and/or the upright post with a hole pattern, you can make both of them extendable to give you maximum versatility.
Mark