ok I will do some test tomorrow. It might be working to put a SD from a RPi4 into RPi3 but not the other way around
Thats what I found on raspberrypi website
Model 4 (B) green LED blinks regular when the PI doesn't detect the SD-card, and irregular when it boots, if the LED stays inactive (with power on) perhaps you need to repair the BOOT EEPROM code