For those interested in updating to ‘307’ version and looking for this patch:
http://download.wdc.com/nas/sq-040400-307-20150921.deb
Naturally 
My unit just upgraded from ‘303’ smoothly in below 20 min time (indexing past initialisation included) and all works fine!
Bit of a background / comments from my side regarding V4 upgrade process:
I did manage to upgrade mine from v3 to v4 succesfuly only quite recently - but only by doing full factory restore eventually and by (carefully) moving my data out of the unit prior to the upgrade / reset and into the cloud aftewards.
My previous updates were frankly disastrous to that extend, that I decided to downgrade manually using this method:
http://community.wd.com/t5/My-Cloud/Poll-Who-has-reverted-to-V3-firmware-and-why/m-p/874980#M34452
To cut the long story short it was probably dud data that literally ground my unit to a halt (it was not able to finish indexing / thumbnails creation, which then in turn was preventing any normal access and work with the unit) was SmartWare backup data - probably something to do with very long file names & access paths.
However I also witnessed myriade of other strange errors (share- and user-related) in GUI, hence my decision to reset the unit completely.
Now’s all dandy and unit works absolutely fine: thumbnails are created when necessary, data rates are relatively high, all anciliary apps (WD Photos) work in order. I also don’t mind interface being like it is now - less gloomy!
One thing though:I did experience Content Scan Complete : 0 files indexed in all categories immediately after first restart when I upgraded to V4 earlier.
The solution that worked for me was as per this thread:
(so: I did shut the unit down and booted it up - and then re-build commenced and completed succesfully within the next few hours)
