Finally got around to finished Elfin Lied and dont get where you got the fact that Lucy only has 4 arms from (the part where she battles Mariko may have hinted toward that but they never explicitly said it). The vast differences between the manga and anime is that ep 13 is basically completely made up. The anime followed the manga mostly till ep 12 (which is around vol 6 of the manga out of 12) so the anime is only really half the story. Sighs i wanted to hear her singing "Elfin Lied" (its a poem/song in the manga [slight mention in the anime] and part of the reason i watched the anime)