what are they for?
You can plug a computer monitor into them if you want an enormous screen that isn’t touch sensitive. I’ve never used them.
HDMI O (output ?) so is to plug a computer monitor.
HDMI I (input ?) what is for?
They are both output, so you can have two enormous screens. It’s what is on the main pi4 so you can’t remove them.
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Technically, you can buy a large touchscreen that can plug into a USB port, and have a very large, touchable screen. It’s why I have another post within the last day or two asking about adding one to v5 lol
HDMI connectors on v4 are outputs: number 0 and number 1 and not I/O 
and yes I totaly agree with
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Nicked and added to the Wiki User’s guide page…
https://wiki.zynthian.org/index.php/Zynthian_Users_Guide#Main_Guides
I’m not entirely sure about the copyright on the base image I thieved btw.
I’ll let mightier forces than me decide…
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