Which version to download?

Since Mid 2019 there are four different Raspbian versions. You can find out the version you're running by typing the following command in the shell:

$ lsb_release -c

This command will output the name of the installed Raspbian version:

For Buster, which is the latest release required for the Pi4

Codename:       buster

For Stretch

Codename:       stretch

For Jessie:

Codename:       jessie

and for the very old Wheezy version:

Codename:       wheezy

Download the correct version on the download page.