Dawid,
Sorry but rekordbox only handles video on channels 1 & 2.
If you're getting feedback issues only when the HDMI is connected to your computer, the problem is likely one of grounding creating a loop. Please consider using some kind of isolator or ensure all items are connected to a common ground and not different electrical circuits.
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Thank you for your feedback.
Are there any plans for changing it in the future ? Will it be possible to play video with 3 or 4 decks?
Do you recommend any particular type of isolator?
What do you mean by common ground? How should I do this?
Thank your in advice :)
DC
No plans for 3/4 channels of video at this time.
I would start by trying to ensure all components are plugged in to the same electrical circuit. Here's a good article on ground loops and isolation.