Now, I certainly am not an expert on naval maritime navigation, Nor can I quote rules and regs at random for FCC part 97 of the Amateur radio service. But being the holder of a amateur radio license,

and based on the diggin' I did on the internet about this,

my reasoning would be..... The four digit number isn't a registration number, it is a call sign. (The vessel has a separate registration number) If the vessel is assigned a call sign, it would stand to my reasoning that it would apply to the radio station as well, since the radio station would use that call sign to identify the ship. So why waste paperwork when the call sign assigned to the ship could be the radio station call sign as well!!! We are talking about the paperwork laden US Government though....

Again, this is just good old fashion BigRed1 reasoning!!
However I do believe one thing is certain about the letter "N" in the 4 digit / letter call sign. Whether it is the ship or the radio station on the ship, it is the international designator for all US Naval and Coast Guard ships.
Captain BJ might have to make a ruling on this one!!!! Either way, I won't have time to claim that RC and Moon Pie, so you are welcome to it!! This has been fun though!!!
Allen