So it's the end of january already. Nearly 3 months went by, Roundcube 1.4.2 has been released, a big update for Obsidian has been released with the update of many third-party scripts, yet no word on this. I can imagine a proper update and testing might take some time, however, the Roundcube 1.4 isn't as big as you think. The major design overhaul might be very impressive, under the hood it hasn't changed that much.
So why is there still no update? There is only one thing that comes to mind... it would interfere with their Premium Email product. You want this new great layout? Pay for it... or you are out of luck. "It will be supported." Yes. But I suspect not before the official support for 1.3.X ends. The more they can wait and squeeze money out of the before mentioned product, they will. Mind you this is off course speculation from my side, but I simply don't see why it otherwise takes this long.
Yes, it is an option to install / upgrade to RC 1.4 itself. But to be honest I was expecting a bit more (as in: a officially supported solution). It's where we, the Plesk users, pay them for.