I watched season3 last 3 episodes [eventhough i had already watched till season3 finale a couple of years back]-- then season4,5 together ------ the story is so well written, extensive and gradually turned from scifi to paranormal/mythical concepts that it takes a toll on your brain!
alternate universe, fighting the observers with stored fringe artifacts, the typewriter, the cow, cortaxiphan, tearing of the veil between alternate universe, collision of the 2 universses to yeild a third universe etc.. the list goes on and on
watch season3,4,5 together --- i thnk alternate reality came at the end of season2