Exporting to Latex PDF does not add a newline at the end of headings level 4.

Source:

* My one
** My one one
Plain text
*** my one one one
Plain text
**** my one one one one
Plain text
** My one two


Latex pdf output, see the inner heading, level 4:

I've read about org-blank-before-new-entry, but it looks like I'm looking for something after new entry of level 4.

Observing the generated LaTeX source, it turns out that level 4 is implemented by a \begin{enumerate} \item, that explains it.

How can I make org-mode add a newline after a header of level 4 or more?

See the H: directive for export settings.
In your case, level 4 is treated as a list item. Increasing H: will give you deeper heading levels.