This is why I am a strong supporter of landing the British fighter in Pearl on UK 1 and why I am almost adamant that Japan must send every possible unit to hit SZ 52 that can reach to avoid being left with Battleship, Carrier, 2 Fighters.
That said, let me get back to the 6 submarines vs 4 destroyers.
Technically speaking, yes, if the battle was SOLELY between 6 submarines and 4 destroyers and some idiot decided to go until the death, then the majority of the time the submarines would win. However, this will never, EVER, happen. There will almost always be fighters involved as well as carriers, transports and battleships!
How many wars have been won with 6 submarines + 1 transport? How many wars can be won with just 1 destroyer and 1 transport? Why? The destroyer can bombard and that can give you land and it is land that allows you to win the game. Submarines are great. I recommend all navies have them because they make wonderful sacrificial units and are one of the better bangs for the buck (Carrier + 2 Fighters is better than 4 submarines and approximately the same cost) but they are worthless without surface ships.
Scenario:
You have 6 Submarines
I have 4 Destroyers, but I also have 3 fighters and a bomber.
You get in range. I sink 2 of your submarines before you can submerge (LL average.) Now it is 4 submarines attacking 4 destroyers and those 4 destroyers will win most of the time.
Why will it work out like this for you? Because you were not strategic enough to consider that your submarines are powerless against enemy air units and that you would have to be in range of enemy air units to also be in range of the enemy fleet.
How do you fix that? Buy destroyers. Or, if you are lavish enough, a battleship and or carrier/fighters to defend your submarines. But now you have ruined the pure analogy you so wanted, the cherry-picked analogy. In other words, the destroyers are the better option because they can defend themselves against submarines and enemy airpower.
And before I am taken out of context, I am not attempting to say you should only buy destroyers. I am saying you need to buy some. 4 is plenty which would mean +2 USA and +3 Japan (assuming you did not lose your starting one.) Submarines can take up the rest of the slack, but you need some and you need to be able to split them up between different fleets if it comes too it.