The trick was to copy NOOPS on the SD card and let that install Raspian. Apperantly Raspian vanilla does not work (yet) on Raspberry Pi 4. I even managed to install elecLIBRE parallel to Raspian, by clicking both at start up. Here is what I did:
I downloaded NOOPS.
I formatted the SDcard with FAT32.
I copied NOOPS from HDD to SD card.
I inserted the SD card into the Raspberry 4.
I started the Raspberry 4
After a couple second a menu comes up with two options:
To install Raspian
To install elecLIBRE
both on that very SD card.
After a couple minutes the install was finied. I started the Raspi new and had again two options, to start Raspian or libreELEC.