Basically there is a seat in the head for the wider portion of the plugs just above the threads to stop on. This seat, notch, ledge, whatever you want to call it is in the same location in the 03/04 heads. What is different is the 03 head threads start at the bottom of the head and go up 4 threads then there is space between there and the seat.
The 04 heads threads start at the same spot, but go up 9 threads much closer to the seat.
So when you screw a plug in your are using the bottom 4 threads of the plug on an 03 head and the bottom 9 plug threads on an 04.
Most plugs have 10-11 threads.
So this is how to find out how many threads your head has.
Make full 360 degree turns on the plug
4-4.5 turns to remove or seat the plug on a 4 thread head
8-8.5 turns to remove or seat the plug on a 9 thread head