Plex originally launched using a preview version of the Google Cast SDK, which is the technology behind Chromecast. Since then, Google has switched to a release version of the SDK, which is more stable and secure, but now requires iOS 6 or newer to function. The original preview version of the SDK is no longer supported.
Starting with version 3.5, the Plex for iOS app has been updated to use the new Chromecast services and no longer uses the preview version. iOS devices running iOS 5 will not be Cast Ready.