As I was tired to switch the SD cards by openning the case, and removing the Pi4 from my Smartipi case, I find the solution.
1) First I made make the RPI booting on the USB with the (large) standard PI OS.
https://www.tomshardware.com/how-to/boo ... y-pi-4-usb
This modify the internal RPI4 eeprom, in order to boot from the USB.
2) Then I made a new fresh standard Portsdown 4 install with the Lite Pi OS on the USB key and it runs as usual.
Dont forget to pull out the micro SD card !
Have fun
Alain F1CJN