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Altough the manual (v10) states TVP is compatible with WDM. Later I found this is only true for input. WDM is exluded for output.(why!?) (WDM is used for generic video input and output on windows )
I'm trying to get a decklink 4k to work in conjunction wit TVP.
Does anybody have any suggestions, or workarounds?

Sorry, but it's the first time I hear about WDM (and I work with video and animation for almost 20 years) and searching the Web so far I found that it's a MS Visual Interdev98 file, used to create Web applications. How did it end up as a video format in your case? Anyway - it is definitely not any standard video format that anyone would consider to use for exporting video.

Could you be more specific about the result you expect to have? Decklink is a capture/playback card as far as I understand, so what exactly is the use of TVPaint here? If you describe your process more exactly, it might be more helpful.

WDM is not a file format, but a means of capturing and sending video to and from external devices. Something like twain, but for video.
(manual TVP 10 page 5-15 > "WDM devices are Windows Device Manager devices (used by most WebCam and DV format camcorders")

I Have a Blackmagic Decklink 4K SDI I/O card (WDM Compatible). I use it with products like After FX, Premiere and Da Vinci, for viewing the output on my monitor.
I would like to do the same with TVPaint. So, work on my computer monitor and have the output in a valid video format on an external monitor. Example for validating color, jaggies ,jitter, line quality and stuff like that.

As is, within TVPaint, the card can be used for capturing, but not for playback.

I can't recommend that method.

Video editing software pre-renders the output (in one layer only), that's why it's possible to send it through a dedicated video card to a calibrated output device.

Animation software (not only TVP) only renders a display view (of all layers) and can only play back in approximate speed (depending on complexity of the scene), so you earn nothing when you put that through to a video monitor. Recommended workflow is to export a video file with the right setting and test this.

Ok, I see now, Slowtiger's answer is correct - TVPaint is not a video processing program, so you should first export the footage, then import it into your editing software, and then check it on your Video output.