differentiate org-level-3 and org-level-4

[ninrod]

right now org-level-3 and org-level-4 have the same colouring. consider using a different color for each one in order to increase readability.

[NicolasPetton]

@ninrod do you want to send a patch/open a PR?

[ninrod]

I'd love to @NicolasPetton, but I'm still beggining to learn elisp! And I trust that you have a way more refined taste for the zerdark color palete.

What do you think would be a good color to cycle between org-level-3 and org-level-4?

[ninrod]

@NicolasPetton, so after your suggestion I've decided to take a shot at a pull-request for this, which turned out to be way easier then I anticipated. ref: #29