Like most others have said, most auto parts stores got 'em all messed up and don't differentiate the 4.0's. Best bet is to do what Cole said, look up a '05 Mustang 4.0. lol.
Don't run Bosch's in it Alex. I ran 'em once in the Ford and ran like garbage and many others report the same thing. Run either straight up Motorcraft stock plugs or Motorcraft knockoffs, Autolites. APP5144 is your Autolite Double Platinum part number. 5144 is the correct plug # for Autolites...prefix and suffix are the different model lines (AP5144 is the platinum). I don't have the Motorcraft number on-hand at the moment.
'01 XLT disguised as a '03 Fx4 Level II
Big. Loud. Slow. Bright.