I've chimed in within a few threads in the Sierra Forum, others are having issues with their 4k displays. This morning I visited a couple of my sub-consultant's offices and was told they are having identical issues. Me, and none of my workers are having problems with the upgrade and our displays; I have a few field offices, and carry a go-bag that includes things like scales (normal people call them "rulers"), a laser level and an electronic level, and, a couple of mDP>DP cables...
I loaned one of my subs the two cables I bring with me, and the "problems" on that Mac workstation disappeared. I'm guessing now that some Macs might be a bit more picky about which cables are being used (I use only Accell cables now, they're on the DP Product Portal - the stock Dell cables by COXOC returned power over Pin 20, so I binned them) and/or there's a bug in the OS? I used several of the Betas and never had an issue on my rMBP and I'm actually seeing more resolution options now on my two P2715Q displays attached to my rMBP. In those threads, a few members related that even TB cables (DP 1.1a) aren't working for them; I've not tried one of my TB cables, but I don't have a need to do so. FWIW, when I swapped out for the Accell cables, our Mac/PC operating temperatures dropped a fair amount and at least one other member backed that up. That's all I've got, good luck...