I bought this to replace my (now previous) chain-checker, the CC-3.2. This newer model compares chain distance between different points on the chain than the prior model (in a Park Tool reply to a question about the 3.2 on their site, they mention the 3.2 checks 'from inside of a roller to the inside of a roller' while the 4 checks 'from the front of a roller to the front of a roller', which '...eliminates roller diameter from the measurement').
Since chain wear is about the pins and not the rollers wearing, this newer gauge is apparently the more trustworthy of the two. For what it's worth, testing both back-to-back on the same chain that had about 1,600 miles on it had the CC-4 indicating it was time for replacement where the CC-3.2 didn't; I'll be using the CC-4 from now on.
Easiest chain wear tool I've used. Make sure you measure the chain under tension. I measure the chain above the chainstay while applying torque to the pedals. In the past I didn't do this and the lengthened chains wore out my sprockets.
I have a more comparative Park Tool for checking chain wear. I bought this one as a secondary check. So far both are giving similar readings.