That says that it is an external graphics card (also called an "external GPU" or "eGPU"). eGPUs are not supported for use with Isadora. (I believe it is related to eGPUs not fully supporting OpenGL.)

From our Isadora 3 System Requirements article under "Unsupported Hardware Platforms": 

Isadora is not supported on:

• Computers with multiple discrete GPUs that have not been properly configured
• Computers with external graphics cards (eGPUs)
• Some tablet devices such as the older versions of the Microsoft Surface Pro
• Computers that run macOS on non-Apple hardware
• Windows computers with ARM processors
• Windows computers running in S Mode or Tablet Mode
• Virtual machines (including cloud-based processing)
• Linux machines

Running Isadora on machines utilizing unsupported hardware configurations may have unexpected and undesirable results in terms of performance and usability.

If you want four outputs from a single device, you can investigate a Datapath fx4 (more expensive but easier to work with) or a Matrox QuadHead2Go (less expensive but harder to work with).

Since you're on Windows (at least the link to the Startech device said it was for Windows) you could also look as MST hubs I believe, but I'll leave giving specific recommendations for MST hubs to someone with more experience working on Windows.

Startech USB32HD4 External 4x HDMI USB

 Hi there Stefan, 

A few questions really:

1) What computer are you using for this; is it a Mac, a windows, and what type of OS version are you on?

2) What version of Isadora are you using?

3) Do you have any output on those HDMI ports when you just plug a display into it? Are the displays recognized by your OS?