Please note that you need to use Unity 2017.3 (not the most up to date one). This means that you will need to download the free Unity editor and make a Unity account to mod Sudden Strike 4. In fact, it will unpack a Unity Editor project to mod the game.
Once you download and start it, it will prompt you with a launcher window that lets you extract all files needed to mod Sudden Strike 4. You will find it in your library under the “tools” section as a unique download called “Sudden Strike 4 – Editor”. How do I access the modding tools? What are the prerequisites?The modding tools are implemented as a separate “tool” project on Steam.