User blog comment:SupremeAssassin/Assassin's Creed IV: Black Flag - Epilogue and Thoughts/@comment-5174994-20140110174005/@comment-5019018-20140111222959

AC3 - 47 main memories

AC4 - 45 main memories

Alright, not much difference in total. But AC4 has more fun sidey-quest stuff to offset the slightly shorter story.