I recently read on the old RYPN.org forums about how apparently, the Southern had restored three of their Ps-4 Pacifics for restoration. However, only the 1401 was ever saved by the Smithsonian due to its association with FDR.
This is from a Wikipedia article, and I read a detailed story about this some years ago that said the same thing. The engine wasn't saved by the Smithsonian. It was the personal effort of the great Graham Claytor (who was also a huge fan of steam) that saved the 1401. I seem to recall that it took some convincing to get the Smithsonian to take it. I never heard that it was saved particularly because of association with the FDR funeral train, although it has some historical significance due to that, and that may have weighed into Claytor's argument to deButts to save that particular engine. The indication was that only the 1401 was saved - all others were scrapped.
" In the 1950s, war hero and outside legal counsel to Southern Graham Claytor (who would later become Southern's president) convinced then-Southern president Harry deButts to donate one of the retired Ps-4s to the Smithsonian instead of scrapping it. In this way 1401 was saved, and has been on display at the Smithsonian since it was delivered there on November 25, 1961."
Also, this:
"When Graham Claytor was a Southern executive in the mid-1960s, he attempted to lease 1401 from the Smithsonian for operational use in Southern's steam excursion program. The Smithsonian refused, and Claytor leased Southern Railway No. 4501 (originally a freight locomotive with a 2-8-2 wheel arrangement) and painted it in the green, gold, and silver scheme instituted for the Ps-4s."
This seems to confirm that there were no other Ps-4s in existence that could have been used.