Please guide me how to download Expression Engine version 3 and 4.
There are at least two ways to get copies of EE 3 and 4.
Log in to https://expressionengine.com. If you have any EE purchases from this site before, you can go to your Purchases page and download the latest 2, 3, and 4 versions of EE.
Clone the git repository at https://github.com/ExpressionEngine/ExpressionEngine/. There is a
master-v4 branch you can fetch and switch to.