I bailed after S2. *shrug*
I just finished season 4.
Season 3 wasn't very good in my opinion. It was way too convoluted and the increasingly off-the-wall conspiracies just got tedious. Season 4 represents a reset, almost a show reboot, as it gets back to its roots, and I'd say Season 4 overall was excellent. There was some off-the-wall sci-fi stuff, but it was contained and moved the story along. The overall writing was tight and the story arc was good. The season finale wasn't as good as I hoped, but it was still good, and the nine episodes getting there were mostly excellent. The new
clone wasn't as good as I'd hoped, but nonetheless she was serviceable, and did add to the story.
BTW, I do think Tatiana Maslany is an excellent actress, but I don't think she's great at foreign accents. To a North American audience I think her poor English accent is passable, but I hear that the Brits cringe at her accent. Similarly, I don't think her eastern European accents are great either. In contrast, many Europeans think Jordan Gavaris' (Felix) English accent is amazing. In fact, I read that Maria Doyle Kennedy (Siobhan), who is Irish, thought thought Jordan Gavaris was a Brit when they first met.
In terms of the actual acting (outside of the accent) I think Tatiana Maslany does really good nuanced portrayals of both Sarah and Cosima, to the point that I rarely think of them as being the same actress. Alison's character is more of a caricature, as is Crystal's, so I would have preferred if they were toned down a bit, but then again both characters are likely caricatured intentionally for comic effect.
tl;dr:
Tatiana Maslany is a very good actress, but doesn't do foreign accents so well.
Season 4 of Orphan Black is a great series reset, but the season finale is not as great as the rest of season 4. Overall though, season 4 more than makes up for the mediocre season 3.