If its the motor mount and you only feel/hear it in reverse, its more than likely the rear motor mount. With that being said when one breaks, the others will soon follow if you don't take care of it quickly.
If this goes on for a while it also stresses the transmission mounts. Its kind of a chain reaction. So, if this is the problem take care of it soon!
I am basing all of this on the fact that you said the "clunk" sound is coming from the front of the car and it seems to be happening around the time you shift into gear. I have had this problem twice in about a three year period because I replaced the motor mounts with an aftermarket product the first time, and they were inferior quality.
Does your car make a "thud" sound when you initially shift into reverse or is it only when you shift into D4?