I have an external USB disk with ext4 unencrypted:

/dev/sda1       3.6T  2.9T  613G  83% /data

I wish to encrypt this disk without losing data. I migrated this disk from Raspberry3 to Raspberry4, because of better performance I now wish to encrypt.

I read about Filesystem stacked level encryption and Block device-level encryption, but it seems LUKS the most secure and the Debian standard.

I read this guide:

WARNING! The following command will remove all data on the partition that you are encrypting. You WILL lose all your information! So make sure you backup your data to an external source such as NAS or hard disk before typing any one of the following commands.

# cryptsetup -y -v luksFormat /dev/xvdc

I can't copy the 4 TB data to another disk, because I have no disk with that much capacity.

How to encrypt this disk without backup/restore the existing data? Is it possible?