@azonenberg option 1: arrange three of them spatially around the fourth across multiple layers, connect the outer three together, treat the whole thing as a weird hybrid between a coaxial cable and a diffpair
option 2: send 3 bits at a time over three of the wires as if they weren't part of a diffwhatever, hold the fourth one at vddio/2 and if anyone asks why you make up some really advanced sounding technobabble