You need to set enable_virtual_whitespace to false in config.4coder. And then bind a key to insert a \t (or spaces) because by default tab is used for auto-completion. Note that if you use any of the auto-indent feature (indent line, indent range), the lines will be re-indented using 4coder rules.
You can change the layout rule by modifying the HookID_Layout in the begin_buffer with functions from 4coder_layout_rule.cpp/h but I'm not sure if it's easy to do what you want.
If you are only editing a regular text file (non flagged as code) you should be able to add indentation without any modifications.