hammy:
I’ve never seen a potentiometer with 4 connections except via google for those with wiper connections doubled up ( in which case I’d guess 2-4 would always be 0k) do you have a picture ?
pin 4 is the centre tap - centre-tap pots allow a differential output without needing external voltage
divider - saves components and board area, or allows injecting a virtual ground into the pot so no
trimming needed.