You have to change the input with 4 inputs like this :
<input type="hidden" name="post_type[]" value="videos" />
<input type="hidden" name="post_type[]" value="e-courses" />
<input type="hidden" name="post_type[]" value="e-books" />
<input type="hidden" name="post_type[]" value="white-papers" />
You may get more detail about a complete code in this article
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