The fastboot mode on the FP4 is reached as described here the button combination you mentioned would not work as far as I know
https://support.fairphone.com/hc/en-us/articles/4405858258961-FP4-Manage-the-bootloader
Alternatively, you can disconnect the USB cable, turn off your Fairphone, then connect the cable and hold the volume down button. After a few seconds, you’ll see the fastboot mode.