4K 60Hz over DP with Late 2013 Macbook Pro 15" + Dell P2415Q?

I have a Late 2013 15" Macbook Pro with discrete graphics.

According to this Apple support page: https://support.apple.com/en-us/HT206587 this model supports 4K 60Hz over MST.

I just purchased a Dell P2415Q monitor, which supports DisplayPort 1.2, MST, and 4K 60Hz.

I have the two connected by the included mini DisplayPort to DisplayPort cable.

And the monitor is running at 30Hz.

Everything I've read suggests that I should be able to get the monitor to run at 60Hz, but I can't find out how to. I'm trying to find answers among all the search results, but they're either outdated, different models, or asking about HDMI.

Can anyone assist me?

EDIT: Here's the monitor's display options, which show that I've turned on MST and where I'm getting that it's running at 30Hz:

Okay so I got 60hz working at 4K. Despite the fact that I swear I already tried this, the key was turning off MST.

Unfortunately, I can't recall why I turned MST on in the first place. Did I create my own problem? I will say that I was unfamiliar with what Multi-Stream Transport was when I read the Apple support doc that I linked to in my question, and the doc was written in such a way to suggest that it was necessary for me to have on to get 60Hz on my MacBook Pro model.

I have a 15" mid-2015 MacBook Pro and had the same problem: my P2415Q monitor was running at 30Hz instead of 60Hz, despite being connected over DisplayPort. I'm not sure exactly what caused this problem in the first place: I had previously been using the monitor at 60Hz. But it seems to have gotten stuck at 30Hz after I unplugged from a different monitor. Initially, MST was set to Off.

I solve the problem by going under Menu -> Display and setting MST to Primary, then to Secondary, then to Off. Initially, MST was set to Off, so it is strange that this fixed the problem, but it did!

Now my monitor is running at 60Hz!