The general consensus with v4.x is the same as with all other single bay My Cloud firmware, there are problems of one sort or another with it. You can read the Change Log (http://support.wdc.com/download/notes/WD_My_Cloud_Firmware_Release_Notes_04.04.03-113.pdf) for the firmware to get an idea of what was fixed in each firmware revision. Unfortunately it won’t tell you what was broke.
If you are never going to use any of the remote access features (outside of FTP) and never will expose the My Cloud to to the Internet, and are not using it as a media server, then why update the firmware if all one is doing is using it as a dumb NAS? If it ain’t broke, why fix it? Leave it be if its working fine.
As the previous poster indicated somewhere along the way WD change the page file size in v4.x which means you cannot use standard Debian programs/services. One has to use programs compatible with 64K page size file format. As we’ve seen time and again people using the v4.x firmware do an “apt-get” hoping to update the packages on the My Cloud and brick (render unusable) their My Cloud because of the incorrect page file size.
The main reason to upgrade is to take advantage of the security updates/enhancements (if any) that WD has incorporated into the later firmware updates.