Every season so far has been excellent, and I'm very pleased to report that Season 4 is probably the best yet. This time it's only 10 episodes compared with 20 each in Seasons 2 and 3; the original Season 4 has been split into 2 seasons, because the writers' strike delayed production. Season 5 (also 10 episodes) will air on Sky 1 starting this January. Because I refuse to buy anything from the Murdoch empire, I won't be seeing season 5 - legally - until late 2009. But that's OK. I can wait.
I think BSG works better with the 10-episode format. I genuinely believe it's almost impossible for TV shows to turn out more than about 10 to 15 good 45-minute (i.e. 1 hour with adverts) episodes of a series in one year, no matter how many people they have working on the show. Quantity seems to trade off against quality every time. BSG Seasons 2 and 3 each consisted of about 7 or 8 classic episodes, 5 or 6 very good ones and the rest were OK filler. Only about 10 episodes max in each series were crucial to the plot. By contrast, Season 1 was only 13 episodes and featured virtually no filler at all. Like Season 1, Season 4 cuts the extraneous stuff almost completely - there's a rather half-baked subplot about a Baltar religious cult to pass a couple of idle minutes in some of the episodes, but other than that, this is all meat and no potatoes.
I won't give away any plot spoilers (the Baltar stuff is no spoiler, believe me) because you need to watch this season yourselves. But I'm just really pleased that, to use a phrase I picked up from Steve's Very True Things blog, BSG has in no way "jumped the shark", whatever some of the numbskull reviewers on amazon.co.uk might tell you. In fact, it may well be that the best is yet to come. More on this same time next year.