I have a USB but not a micro SD card. I installed the operating system onto it.
Can I use that as the storage and OS layer for my Raspberry Pi 4?
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Since you have a Raspberry Pi 4 it is possible to get it to boot from USB.
The process to switch from SD card boot to USB boot is demanding that you FIRST boot from a SD card.
So YES you need a SD card.
The process is roughly:
Create a bootable SD card with "Raspberry Pi OS"
Edit the /etc/default/rpi-eeprom-update
Install the bootloader update (That have the USB boot capability)
Create a bootable USB media
Now you can boot your Raspberry Pi 4 from the USB.