Add a class to posts in increments of 2

Add a class to posts in increments of 2

Is it possible to add specific classes to posts/pages in a query in increments of 2?
For example
Post 1
Post 2
Post 3 (add custom class)
Post 4 (add custom class)
Post 5
Post 6
Post 7 (add custom class)
Post 8 (add custom class)
Post 9
I don't mind using jQuery if needs be.