How to tell the 4 and 6 rear calipers apart? Thanks Rick

The pads are slightly bigger. I think they are the same size as a stock 914-4 front pad.
The dimensions are in the factory manuals.

go on pmbperformance website and see the differences. don't buy a set of 6's from someone without visual confirmation on what your buying. both bolt on but the price and size are different.

The -6 calipers have a smooth spine and the -4 calipers have a casting hump on the spine.

-6 use a FMSI D31 pad same size as the early -4 front and the -4 take a FMSI D30 pad.

Thanks guys,Rick