I'm trying to display subcategory files in the main category like I was able to in IPB3.

Even on the marketplace they aren't doing it so I'm not sure if there's a features to do it yet.

If you go to the marketplace and click on "Previous Versions" under "Categories: you will see exactly what I'm talking about.

Link: https://community.invisionpower.com/files/category/158-previous-versions/

If anyone knows how to display subcategories in the main category please let me know. Thank you!!! 

Not possible. According to IPS, leave a feedback on Feedback forum... just don't know how many feedbacks are required.