I'm running Sonar 8.5.3 Producer Edition and trying to create a groove clip. Bouncing 4 measures of Perfect Piano midi to audio works fine, everything syncs nicely. When I select the audio that has been created on a new track and apply Groove Clip Looping to it it gets stretched to 5 measures. 
 
I know it must be something dumb I'm doing or not doing but I can't figure it out. Any ideas?

You're not doing anything wrong. Groove Clip Looping just does that sometimes.
It tries to guess how many beats are in a clip, and usually guesses wrong. It's an easy fix, though.
Open the Loop Creation view.
If your song is 4/4 time, we know that 4 measures should be 16 beats.
The Beats In Clip window probably shows around 20 beats. Change this value back to 16, and you should be good to go. You may need to play with slicing to get it spot on.
For what it's worth, MIDI groove clips are much more accurate. It's easier to assemble your loops in the MIDI track, then bounce the complete track to audio. 

Thanks for the reply, it got me looking more carefully at what I was doing. It seems the audio clip being converted to a Groove clip needs to be clean. If I use the step sequencer to create the midi track, then bounce that to audio and groove clip it all is well. If I use a clip with lots of other instruments, syncopation, non-quantized notes, etc it gets all squirrely and the results are unpredictable. Reaper does a FAR superior job on such.
 
Anyhow problem at least identified and since I always do all my editing in midi first it isn't an issue for me. I was trying to help a friend, he will continue to use reaper for his audio stuff.