I would say 6 to 8 is average and 4 weeks is smoking fast. Also depends on how many allocations your dealer has, the priority of the order. Once ford picks up the order and you get a VIN the process is fairly fast. The biggest issue I would ask the dealer is if they have an allocation for your order. I ordered from a small dealer and waited 13 weeks to have my car delivered, I would recommend a larger dealer if you're in a hurry. Also check out the below link for more information. Good luck
http://www.allfordmustangs.com/forums/2011-mustang-talk/363876-2014-mustang-ordering-questions.html
http://www.allfordmustangs.com/forums/2011-mustang-talk/363876-2014-mustang-ordering-questions.html