Lets look at the facts, ACIV is a HUGE game with a myriad of different activities to partake in like harpooning, hunting, diving for treasure, battling ships on the open sea, crafting new gear, upgrading your ship, boarding and plundering ships, collecting sea shanties, finding treasure, using treasure maps to find buried treasure, side missions, assassination missions, privateering missions, collecting Animus Shards, board games to play
...and a huge ocean to explore with many islands to discover each with their own secrets.
And overall mission variety in which missions feel like part of the sandbox rather than Call of Duty segments like GTAV
I'd say it outclasses GTAV easily in the sandbox category. (Plus it has a multiplayer mode that actually works)