Officially, no you cannot install Plex to the v4.x single bay My Cloud. Unofficially it may be possible but it involves using SSH and modifying or changing the v4.x firmware.
There are several prior threads that discuss how to stream content to the Chromecast. Use the forum search feature, magnifying glass icon upper right and search for “Chromecast”. The easiest method is to use the WD My Cloud app for Android (if you have an Android device). See my following post where I explain how I stream media from a My Cloud to the Chromecast.
https://community.wd.com/t/using-mycloud-to-stream-to-chromecast/172050/2
And here is one unofficial way to get Plex running on a single bay v4.x My Cloud. It involves replacing the My Cloud OS via SSH.
https://community.wd.com/t/using-plex-media-server-with-my-cloud/94429/12
