Is there a 64-bit Raspberry Pi OS?
Raspberry Pi OS is now available in 32-bit and 64-bit editions. Not all Raspberry Pi are compatible with it, but with recent models, a 64-bit operating system improves the program performances.
Can Raspberry Pi 4 run 64-bit OS?
Raspberry Pi 4 has an 8GB version which is the biggest beneficiary of the 64-bit OS. If you used the 8GB Pi 4 with the 32-bit OS, you could still access all 8GB of RAM, but with the limitation that each process is limited to using 3GB each. With the 64-bit OS, a process can access all 8GB at once.
How to install 64-bit Raspberry Pi 3?
Installation overview
How do I download Raspberry Pi OS?
Downloading and Installing Raspberry Pi OS