When it's disassembled to this extent, will you later regret you didn't replace the other three...
If an engineer was in your shoes, he would check the piston-to-cylinder-wall clearance for the three remaining cylinders, compare them to the factory service manual tolerance, compare them to each other, and to the specified clearance of the replacement. The existing piston skirts may have wear resulting in excess clearance even though the cylinder walls look good. If all measurements were in tolerance, replacing piston rings while you've got it apart is advised.
If a shop was performing your repair under warranty, they most likely would replace only the damaged cylinder, reassemble, and send the vehicle down the road to run many miles.
I would probably replace all four if it was mine; guess it depends on how long you intend to keep your CRD and how OCD you are
