There is 1 known 4 word shout, that of which only Miraak has been known to use. Ziil Los dii du. But what if there were more? My theory suggests that there are more shouts than Dragons! Here's one I made earlier:

Lok Toor Du Al

Sky Inferno Devour Destroy

It rips the sky open like a pack of Fries, and is capable of destroying or scarring the world permanently. 

Heres another:

Gol Kref Raf Dil

Earth Bend Hole Deep

Creates a massive hole in the ground, swallowing all that cross it.

Do you have any ideas? If so, feel free to post below!