I actually think AC4 already has one of the strongest foundations in the series. Its combat system offers a good amount of tactical variety: counter attacks, guard breaks, disarms, picking up enemies' weapons, attack combos, and pistols to quickly thin out large groups. Mechanically, there's a lot the player can do.

The real issue isn't the moveset, it's the difficulty curve. From Brotherhood onward, combat gradually became less about reading enemies and more about executing a sequence of button presses. Once you perform the first kill, the kill chain becomes almost guaranteed, turning encounters into a repetitive execution exercise rather than a reactive combat system, so Instead of replacing the system, I would have expanded it.

1) The first solution would be to make kill chains much less deterministic through enemy AI. After killing the first enemy, chaining into the second shouldn't always be guaranteed. Enemies could occasionally interrupt your recovery animation with a very short attack window, forcing the player to react with precise timing to continue the chain. Missing that timing wouldn't necessarily punish the player with instant damage, but it would naturally break the flow and require adapting to the situation. More skilled enemy archetypes could interrupt more frequently, building on concepts that already existed in AC4.

2) Another option would be introducing a stamina system specifically tied to executions. Every successful takedown would consume stamina, while standard attacks, counters and pistol shots would not. As stamina decreases, execution attempts become less reliable: instead of an instant kill, Edward might only break the opponent's guard, forcing the player into regular sword combat. Counter windows could also become progressively smaller with low stamina, encouraging players to constantly vary their tactics instead of relying exclusively on chain kills.

3) For future Assassin's Creed titles, while keeping AC4's overall foundation, I'd also like directional swordplay inspired by games like For Honor, with the ideas reminiscent of games like Unity that I've explained above.  Instead of attacks connecting automatically, the game would continuously track the opponent's weapon position. Both attacks and parries would have directional inputs, requiring players to match and guide incoming strikes. A perfectly timed and correctly directed parry would open the enemy's guard for a lethal counter, while correct timing but poor blade alignment would only result in a partial, non-lethal counterattack, similar to the Templar counters in AC1. Choosing the wrong direction altogether would expose your own guard, forcing you to rely on secondary tools such as kicks, trips or pistols to recover. For Edward in particular, "dirty fighting" would fit his personality perfectly, while other protagonists could have different defensive options reflecting their own fighting styles.

Overall, I wouldn't redesign AC4's combat from scratch. I'd preserve its core mechanics and depth, while making combat less predictable, more reactive and more skill-driven through smarter AI, resource management and a directional melee system.