The first few episodes were utterly confusing until we realised that the events at the end of Season Three had logically (insofar causality weirdness can ever be logical) knocked-on into Season Four. There is still a fair bit of jumping about between universes but the entente-cordiale brings a new, cosier dimension to the relationship. Of course there has to be a baddie and the reincarnation of an adversary from an earlier season (Season Two I think) fills the void left by Walternate & the shape-shifters while the previously mostly-benign Observers take on a far more sinister edge.
The masterful writing and plot & character development continue to be the real stars with the mad-science gluing the narrative together. It is not possible to describe any of the plot without spoilers but suffice to say that a certain amount of concentration is required and even the few stand-alone episodes reveal a new intriguing twist so an unencumbered memory is essential. There is one episode towards the end of this season that seems to be totally unrelated to anything else but, surprise, surprise it does eventually make sense and sets the scene for Season Five (again, like Season Four, rumoured to be the last).
The cast continue to be superb but, again, John Noble shines brighter than the rest - he should definitely be in the running for an award of some sort. Fringe persists as one of the most original and downright strange series ever to grace our screens. Excellent, mind-bending stuff.