Bought a 2004 F150 crew cab, 4x4 with the 5.4 engine last week. While moving at basically idle speed the engine has stalled several times. The engine is warmed up and does not run rough prior to stall.

 

I just had an identical problem with my 04 S'screw. Returned it to the dealer, they found a technical service bulleting describing the problem. TSB 17646. I believe they wound up re-flashing the control module, but it's hard to tell from the gibberish on the service sheet and I haven't been able to find a description of the TSB online. This appears to be pretty widespread, found lots of similar reports of engine stalls on 04 F-150's. I'm betting this turns into a re-call.
Long & short, get it back to your dealer, ask them to reference the TSB above. Best of luck.