This code works for me if I go to upload a file and choose the Gallery or Media Library tabs then it correctly hides elements on the page.

If I go to the From Computer or From URL tabs, nothing is hidden!

If I take out the if statement then it works. I can't see why it shouldn't be working on these 2 tabs as the URLs are the same apart from the tab parameter and the if statement is only checking the post_id:

Gallery tab works: /wp-admin/media-upload.php?post_id=4&type=image&tab=gallery

From Computer tab doesn't work: /wp-admin/media-upload.php?post_id=4&type=image&tab=type

Any ideas?