Q: Spanning Keynote Presentations Over Multiple Screens

I want to span a keynote presentation over multiple screens. I tried everything that came to my mind:

On my MacPro (late 2013 / AMD FirePro D500 3072 MB) I connected 4 HD capable displays. On 3 of them I want a keynote presentation spanning over the entire screen real estate. On the 4th screen I want the controll screen.

I went into system preferences and unchecked the "every screen uses different spaces". But still keynote would only start the presentation on one screen.

Then I went ahead and connected my trusty triplehead2go DP, using only display port connections. Only to find out that the maximum resolution I can do there is 4080x768px, so the presentation would be "scaled up".

Obviously MacOS is capable for example to run Unity games or processing sketches over multiple screens in span mode, but for some reason Keynote cannot do that. Which is a big bummer!

Is there any thing I might have missed?

Thank you for all the hints!