I loved the last season to bits. The first one is cute but not very organized and I had mixed feelings about it. The second one I downright disliked, but partly because I imagined all the cheap cop outs at the end of it would stay. Instead, season 3 brought everything home to roost, dealt with the consequences of s2, had a tightly plotted arc, and was fantastic to watch. (Bear in mind here that my favourite AtS season is 4.) It would have been a good point to end the show on, but so far, I'm mostly liking what I've seen of s4.
However, if you want to watch it, you have two options:
1) Watch season 1, the opening ep of s2 plus the last two eps (those are really important), thus sparing yourself the misery and inadequacy in between and still getting the important stuff, watch season 3 entirely, or
2) Start with s4 right away. Much as you, say, could start with s3 of Spooks at the point where Adam comes in without knowing the whole Tom Quinn saga, so to speak. Otoh this robs you of the breathtaking experience that is season 3 unspoiled, so better take option 1.
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However, if you want to watch it, you have two options:
1) Watch season 1, the opening ep of s2 plus the last two eps (those are really important), thus sparing yourself the misery and inadequacy in between and still getting the important stuff, watch season 3 entirely, or
2) Start with s4 right away. Much as you, say, could start with s3 of Spooks at the point where Adam comes in without knowing the whole Tom Quinn saga, so to speak. Otoh this robs you of the breathtaking experience that is season 3 unspoiled, so better take option 1.