I do not believe that there will be a "Kindle 4". Amazon is planning on releasing an Android tablet in two different sizes: 7-inch and 10-inch for this Fall:
Cnet article. This tablet will provide users with access to Amazon's Cloud Computing servers; music store, book store, android apps,.. This Amazon tablet will be "Kindle 4". Also, I believe Amazon MAY release an e-reader with
E-ink Triton (colour). However, the Triton films will not be ready until next year