XiaNaphryz
LATIN, MATRIPEDICABUS, DO YOU SPEAK IT
I haven't played P4G, I don't think you need to finish it before playing P4A.
P4A's story is about 20 hours long if you don't skip stuff, but I'm really enjoying it. Only have Akihiko, Aigis and Elizabeth left to go.
Oh yeah, you can likely go in blind without having playing the previous games and still pick up on the story pretty easily. If you have played through though, you'll catch references and certain scenes will have a lot more impact than others.
Elizabeth's story will also make a TON more sense if you played through P3 and P3 The Answer.