After clearing the final Episode 3 story mission, a message implies you can go on to Ep4.

After clearing the final Episode 3 story mission, a message implies you can go on to Ep4. However clearly this isn't the case at the moment.

If it won't be accessible for a while, it might be good to put a different message there or temporarily hide the "To Episode 4" menu option.

The post-Episode 3 content, for Episode 3 (to transition into Episode 4) after the credits was actually released during Episode 4. For whatever reason. (I was told this on this very forum.)