I had a RV-4 and never had the cracking problems, but at my air-field we had about 10-15 RV's at one time or another, most RV-4's. Two RV-4's developed the cracks on the lower mount attach fittings. These planes where used in mostly for aerobatics, dog-fighting and formation flying. They all had 180hp c/s props.
I also never heard of the upper ones cracking. I can't give you more info. Van did change the design of the lower ones for sure but don't recall if there was a design change to the upper ones.
The repair involved removing the engine mount with engine still attached. Drill out the mounts and install new ones (easier said than done). Sorry I can't help or lend a hand, but at least there is a data point or two. G