Control System 4 with just a keyboard?
While it is reasonably possible to use a classic Mac without a keyboard, using one without a mouse requires a System software disk with the Easy Access control panel installed.

With Easy Access installed, you can press Command-Shift-Clear to toggle Mouse Keys at which point your numeric keypad can be used to control the mouse cursor and the computer will be frustrating, but functional.

A limited number of shortcuts are still documented on Apple's site in Archived Technical Note 28141. Also, you can select icons in open windows by typing the first letters of the icon's name (for example, 'sys' will select "System Folder"), followed by Command-O to open.