Other than a different appearance/configuration, the Mk 3 has the two separate sockets vertically arranged and the Mk 4 horizonal.
The method of connecting the cables has been simplified too. No insertion tool necessary. No fixing screws on the Mk 4 faceplate, two "squeeze in" tabs on each side and pull off for access to the test socket and phone extension connections directly behind.
I've had both types fitted with no apparent vdsl perfomance difference. I'm sure there are possibly other changes too.
Three mobile, 1,2,3, PAYG. https://www.three.co.uk/Store/SIM/Pay_As_You_Go