Sadly, I think not; they have too many different operating systems to track changes to, etc…
That ‘rc5’ version identifier is bit of a giveaway that it’s a release candidate…
The samba people are currently working on 4.5.0rc1.
RAC
Maybe WD has a license to one specific version of Samba, like I suspect they do with Twonky, and that license doesn’t include future updates. Would explain why both the Samba and Twonky server versions appear to be years out of date and lack newer fixes and updates.
But in any case the code development track is probably fixed and introducing an updated Samba (or Twonky) would be a time consuming process involving code testing that would upset the current project track release schedule.
But that would be for a released, production version, not a release candidate.
And Samba is free software, covered by the GPL. so not licenced.
Sometimes a license might include release candidates.
What does the “L” stand for in GPL. When one uses Samba they are agreeing to the GNU General Public License (GPL on Samba.org). 
So in effect there is a “license” for Samba even if one doesn’t pay for it.
There very well may be valid reasons for why WD is not upgrading Samba to later versions. Until they communicate those reasons to us we are engaged in speculation that very well could be (and probably is) wrong.
WD has made several decisions with the single bay My Cloud models and their firmware that leave one scratching their head and asking “what were they thinking”.
Hail, fellow pedant… (I was pretty sure someone would pipe up). It’s a licence, but it’s a very unrestrictive licence, apart from requiring you to release any changes you make to the code. It certainly doesn’t limit you to using a particular version of code.
Whatever their reasons, I think that using a release specifically marked ‘not for production use’ is pretty remiss. My guess is they took a snapshot of a Debian Git repository at some point, which happened to include the rc variant. Which does kind of smack of poor software configuration management; if you’re using open source software for your product, know what package versions you are using, and ensure they are stable releases, and keep track of developments in those packages, especially security developments, if your product is connected to the internet.
