By default the Raspberry Pi 4 boots from SD card. If you want an image on an USB stick or SSD connected via usb, you can easily do that since images V35 with Repetier-Server 1.4.9.
The easy part is installing the image on the USB device. You follow the same instructions as for SD card, just with the USB device as target.
With some luck you can now just insert the USB stick and remove the SD card and reboot and you are done.
If this does not work, don’t worry. Your Raspberry Pi is just not configured to boot from USB. So burn same image on an SD card, insert it and boot. After then login with a command terminal like powershell on windows with
ssh pi@RepetierServer.local
enter the default password raspberry and execute
sudo rapi-config
Select Advanced Options, than Boot Order and last USB Boot. Leave the program and let it reboot to store new boot order in eeprom.
Shutdown your Pi, remove SD card and boot again.
Also USB boot is preferred it will none the less boot SD card if you leave the SD card inside. The reason is that the boot partition defines which PARTUUID the root directory has. But both SD card and USB share same PARTUUID because they come from same image. In that case the SD card wins selection. If you want SD card in parallel, change the SD card partition and make the SD card partition 1 file cmdline.txt have root= paramater set to that id.
Always remember that first startup will expend the root filesystem after the next automatic reboot and expension will take some time to complete depending on device size. This happens while the system is running, so let it at least run 10 minutes to finish expansion for sure or you might end up with not fully expanded disk in the end.