If I want to run Ubuntu Server on a Raspberry Pi4 can I encrypt the SD card like an encrypted disk? If not is the next best thing to encrypt the home directory?
Yes encryption is supported.
However, the Pi4 bootloader cannot currently (late 2021) handle the normal Ubuntu Server installer. You must install LUKS and run cryptsetup manually after booting your newly-flashed system.
If you don't know how LUKS and cryptsetup work, review a few tutorials and perhaps practice in a VM before encrypting your real system.
Reminder: If booting from an SD card, recall that they wear out. The moment the SD card dies your encrypted data becomes forever inaccessible unless you are prepared.