the easiest way to figure it out is with the 4-link calculator I posted above
but it isn't too hard to understand, it is the point which the axle pivots around so:
if I am thinking correctly
a 3 link wishbone roll center is the pivot point on the wishbone frame.
a 4 link where only one set of links are triangulated, it is the point where those links converge.
a 4 link where both links are triangulated, I don't have an answer, my understanding is that it still gives you a high roll center,
but it seams like it would have to be in between the two convergance points. I need to play with the calculator to understand this.