Are you trying to put Zentyal 4.xx onto Ubuntu 15.xx? That's not going to work, ever, no matter what, not even kind of.
Besides that, any time you install Zentyal on top of Ubuntu you CANNOT do any updates/upgrades to the Ubuntu server. You have to install the Ubuntu server, NO UPDATES ALLOWED, then add the Zentyal repo to the sources.list.
Once you get that done you run "apt-get update" to grab all of the Zentyal stuff and then run "apt-get install Zentyal" and hope it works.
Technically you're supposed to kill pretty much every other repo in your sources.list but a couple of them get to stay, can't remember which ones are which though.
Even more technically speaking than that you're actually supposed to perform the Zentyal installation off-line, which never really works.
Yeah, I know the documentation kind of sucks about it and it's kind of sketchy getting your head wrapped around everything Zentyal, trust me I've bugged them about their documentation a million times now.
Their commercial product documentation is less helpful than this forum to be honest, but that's something I found out a long time ago. If you don't install Zentyal from the downloaded ISO you're not going to be happy with it at all.
Good luck getting things figured out and if you do get something working please post back and let us all know how you did it!