Following on from the distilled brilliance of series 3 (to my mind the finest series of Buffy or Angel ever to hit our screens) I guess the writers of Angel thought they needed to do something grand and impressive to top it; so far as the grand and impressive part is concerned they succeeded, but not without losing some of the magic. This series, more than any other of Angel, is dominated by one story arc which occupies almost every episode from episode 7 (Apocalypse, Nowish) onwards, and although this storyline is both engaging and exciting, it would definitely have benefited from a few more stand-alone episodes, both to break up the dense plotting and to perhaps provide some light relief from the all-encompassing darkness of the main arc.
It's obvious that the writers still have what it takes to write a good stand-alone episode, as Spin The Bottle (it's telling that this is Joss Whedon's contribution to the series) is one of my all-time favourites, but others are practically non-existent (apart from maybe Players) once the main arc kicks in. Don't get me wrong, however; the arc provides some of the most impressive and exciting "plot" episodes of Angel's run, my favourites being Soulless, Orpheus and The Magic Bullet (where Fred gets a welcome chance to take centre stage).
The character drama is another slightly weak point for me in this series, as the whole Cordy-Connor thing, while essential to the story arc, is also rather irritating (though believable, at least from Connor's point of view), and the downturn in Fred and Gunn's relationship (starting from the episode Supersymmetry) is well-written but difficult to watch, and typifies the underlying feeling of frustration which pervades this series.
I ought, however, to draw special attention to the brilliance of Alexis Denisof in this series, as for my money there is no greater achievement in the Joss Whedon canon than Wesley's tragically convincing descent into darkness; to look at the excessively uptight and rather clownish figure who arrives in Sunnydale in Buffy series 3 (a version of Wesley brilliantly resurrected in Spin The Bottle) and then at the tortured husk of a man who's made his mistakes but been unfairly punished for them one would hardly believe they were the same person, and yet never once did I question the writing or the performance which got us from one to the other.
Looking back on what I've written thus far I realise that it sounds as though I don't really like this series, which is not the case at all, as it's still an excellent addition to the Buffy/Angel cannon with some classic episodes and a gripping central storyline.