As of the time this article was written, the options for creating a 4K alpha key using Blackmagic hardware were rather limited, as hardware options that directly support 12G signals are very limited. If you have a device that accepts a 12G signal, then you should simply be able to use Output A and B on the Ultrastudio 4K Extreme, and that should output your 4K key and fill. More than likely, you will need to do some conversion so that you can actually input a Quad SDI signal for your switcher to set up the key properly.This already assumes you have both the ProPresenter software and the alpha key module purchased.
You will need to ensure that your switcher can support the 2SI format of 4K signal transmission, which is where the 4K signal is divided into 1/4 resolution versions of itself and sent over 4 separate SDI lines.
In order to create a 4k alpha key, you must use the Ultrastudio 4K Extreme (or Extreme 3), the only difference being that the Extreme 3 is Thunderbolt 3 while the Extreme is Thunderbolt 2. So make sure you purchase the right one for the type of thunderbolt connection you have. The Ultrastudio needs to be set for single link SDI output so that it is outputting the full 12G signal in 4k. Output A will be the Fill signal, and output B will be the key signal.
You will then need 2-Teranex Mini 12G-SDI to Quad SDI converters. You will take Output A from the 4K Extreme to one box and output B to the other. This will enable the key to be sent over one box, and the fill over the other. In this setup, you will be using the 2SI (Two Sample Interleave) format for transmission of 4K, which will transmit 1/4 the resolution of the 4K image on each of the 4 SDI lines, and not the (4QD - Square division quad split) as this is the only current option for a 4K key using Blackmagic devices. Make sure that both Teranex Minis are set for Quad Link SDI output.
After feeding the two boxes the correct signals, you would then need to run the 4 signals for key and 4 signals for fill into your router or directly to your switcher so that the key can be fully set up in the switcher.
4K equipment and workflows are ever changing as the industry continues to develop them. As such, there may be newer options that are available in the near future that make this easier. As of March 2018, this is the only way aside from direct 12G output we are aware to make this happen.