Wait, what? Isn't Lightroom 5 out now? Yes, it is.
Normally I wouldn't cover this previous version bug fix update in the news section, but it does have a mirrorless tilt to it: the Samsung NX300 raw support in Lightroom 4 was incorrect, and this update fixes that (as well as another small issue). If you have an NX300 and are using Lightroom 4, purge your current cache (Preferences/File Handling/Purge Cache), update, and rebuild your image previews.