The first thing I noticed when I selected the dropdown for modules was that many were missing.

Where are the other core modules??? They appear in the correct folder:

I am not sure why this happened, but to fix it, I simply unzipped DotNetNuke_4.0.0_Install.zip to a new folder and using Host > Module Definitions > Upload New Module, uploaded each module found in \Install\Module

After uploading the modules and installing them, all seems well and I successfully added a new Text/HTML module into the Home page and edited it without incident.

UPDATE: Read Cathal's Comments