There’s nothing wrong with the key I have now, I was just wondering if this is possible.
The 2019 navigator as far as I know uses PROX remotes (keyless entry and keyless start) with a fcc id of: M3N-A2C940780
The 2004 navigator uses a completely different style of keyless entry remote with a fcc id typically of CWTWB1U331 or CWTWB1U511.
If you want to try, go to your 2004 Navigator. Insert key and turn ignition ON to OFF 8 times ending in the ON position. Your locks will lock then unlock. At this time hit the lock button on the 19 Navigator remote. If it programs you will hear the locks lock/unlock again. Then turn key off.