What are the advantages of 4 arm cranks for road bikes?
Predominately a weight thing. The Shimano marketing spiel / hyperbole:


  With a unique four-arm spider and Hollowtech II construction putting
  strength just where its needed, the FC-9000 chainset sets new
  standards for stiffness and low weight.




^^^ Shiny :)

That and because I think it looks distinctive and is seen as innovation. Some people don't like the look, but weight weenies will snap this up.

On the web I found the following comparison:
(weights are combined chainset and bottom bracket)

DA-7900 725g  &lt;-- previous Dura Ace version
DA-9000 683g  &lt;-- latest


That's a whopping 42g, or 5.8%. I'd buy DA if I could afford it, but not for this reason. I'd be better off eating fewer pies.

As with other Shimano releases, the technology has already trickled down to Ultegra and 105.

I guess if you don't need 5 arms, why wouldn't you just have 4?

Shimano is just an example. As andy256 commented above, Campy also has this and maybe others.