There are people who can play both big positions(Horford), guys who are fairly clearly 4's(Millsap) and clear 5's(Drummond). Most teams are going towards the slightly undersized shooter at PF where they can, as the shooting value more than makes up for the defensive loss when there is a good defensive centre on the court.
There are cases where players are centres in size and (lack of) mobility, who are stretch 5's(Hawes, Antic types) and short guys that can't shoot who are undersized centres(Blair, probably Stokes in the draft). That makes position a bit tricky, as there is some fluidity between positions, but overall they are splitting out.
The question for the Spurs is that, if the spurs sign a 4th big, who does that guy defend? If Splitter is on the bench in the 20 minutes high quality backup/occassional starter, he can guard either, but he's probably more challenged by backup PFs that backup C's, so a PF makes sense to me. With that PF, you want shooting, passing, and being mobile enough to adequately defend mobile types like Millsap.