Hard on the heels OS X 10.10.3 the 10.10.4 public beta launched this afternoon. I'm not sure what the changes are and there was zero information about the download other than the fact it was available. As the newest feature of Yosemite I was curious what changes, if any, there might be in Photos. So far all I can tell is it has gone from build (208.52.0) to build (215.1.0) but I have not yet ferreted out any notable changes. I will let you know if I find any.
If we knew what it was we were doing, it wouldn't be called research, would it?