Thank you KM0201.
I got a lot further with you help. So i did the following (in that order:
login as su
copy & paste Box 1
Code
apt install gnupg2
apt install wget (didnt work, was already installed)
wget packages.openmediavault.org/public/archive.key
apt-key add archive.key
copy & paste Box 2
Then i got some warnings and errors.
no IPv6 possible and the following:
https://pastebin.com/UT0xUFMe
But the main information:
Its looks like the first time OMV 5 on RPI4 (4GB) without any errors!
Great!