But why I get only 2 mixed channels in Isadora with a aggregate Device with 4 Channels (see pictures above)?

 I think because of this: 

But why I get only 2 mixed channels in Isadora with a aggregate Device with 4 Channels (see pictures above)?

If I play the audio file with another software (e.g. iina) I get four different channels

Ah, see, I was wrong. I had just checked the hidden inputs on the Movie Player for that exact reason, but overlooked "audio device" and thought the one I was remembering was "snd out". After reading the description for "snd out" I decided  I must have remembered incorrectly and that there wasn't a hidden input that could help 

I could have four movie players, each with a different sound output. But they were not Bluetooth devices. If the Bluetooth devices all appear in the list of sound outputs, it should be possible to have 4 video player with each a sound output.

Hi, is it possible to route the audio outputs of 4 movie players to 4 different audio outputs in Isadora, without any external hardware? 

Unless I'm mistaken (and I certainly could be) it seems to me that even if there was multi-channel audio routing in the Movie Player actor right now it wouldn't be possible to do what you want because what you're talking about is multi-device audio routing.

If you could get the Bluetooth speakers connected all at the same time, but able to appear as separate audio devices, you could easily do it on macOS with sound files like this (just four separate AUAudioFilePlayer actors going to AUAudioDeviceOutput actors set to different devices):

Not sure how you'd do that though, since everything I've been searching up online about connecting to multiple Bluetooth speakers says you'd need to group them into a Multi-Output Device in Audio MIDI Setup (which means they'd all be sent the same thing instead of being individually addressable):

I could make a "Hausgerät" with 4 channels but in Isadora I get only two mixed channels

I haven't tried personally but it seems to be possible:

https://www.youtube.com/watch?v=SBrQ3Xduhk8. (German)

Edit: Sorry I haven't seen you want 4 different channels. 

Best regards, 

Jean-François

Interesting, i want to know the same thing. Did you find a solution?