But regardless I’ll play along.
If all 4 attacks hit it doesn’t matter.
If no attacks hit it doesn’t matter.
If 1 hits nothing changes.
It does matter, first as you said above you can't just take the average damage, there is a probability distribution at play.
Also this is a decision you make AFTER your first attack hits and AFTER you do damage with it bit before your second attack. So there is no possibility of no attacks hitting. As a PC with 2 attacks you only have the option of focusing your fire after the first of your two attacks hits.
That is the decision point and whether the next three all hit or none of the next three hit there is a statistically "best" decision on whether to attack the same enemy again or attack the other enemy.
Likewise after player 1s turn whether he hit one of them or both of them there is a statistically "best" decision for player number 2.
So really we are only looking at 2 and 3 attacks hitting scenarios.
No the first scenario I am looking at you hit the first guy with your first attack and 3 attacks are remaining. This could turn out be a 1-hit (3%), 2-hit (19%), 3-hit (44%) or 4-hit scenario (34%) but it can not be a 0-hit scenario if you hit and sapped one enemy.
The chance of a 2 or 3 hit scenario is the most likely outcome. There are also multiple variations of 2-hit and 3-hit scenarios depending both on who you attack and whether you or your ally or both miss one (or your ally both) attacks.
So we have the following scenarios.
You hit twice and your ally none.
You hit twice and your ally once.
Given you hit twice then if your ally hits none you’ve either killed an enemy or given it disadvantage. Depending on the chance to hit you and their number of attacks disadvantage could be better or not. Given their low attack applying disadvantage to both would be better than killing one.
If you are assuming average damage yes, but it is 4 times more likely that you hit twice and your ally once than it is you hit twice and your ally misses both. For the extra guy that survives to overcome this you would need an extremely low AC for this to be a better decision.
Of course I’d count this example in the 1% I already acknowledged it’d be better for. So Not really sure the point here. 1 anecdotal cherry picked example only proves not always, but I never said always.
I think it is much more than 1% and if you are using a sap weapon with extra attack you are getting no benefit from the sap property if you target an enemy you already hit. Basically it is like having no mastery after the first hit (unless you are a high level fighter).
Further if focusing max damage on one enemy is always (i.e. 99%) better, then logically Vex, Graze or in most cases Topple are all going to be better than sap in 99% of cases, because these all substantially improve your damage on the enemy you target with extra attack. There will be a positive damage boost from these masteries which you do not get from sap. You are giving up damage against a single enemy to use sap at all, and if giving up any damage to a single enemy is never worth causing disadvantage on an attack, then sap would never be the best mastery to use.
And again, you’ve cherry picked one of the 1% examples I already agreed existed. What point are you trying to make?
I picked examples that are easy to illustrate, but it is a lot more than 1% in my experience. As a matter of fact I think itusually better to target a different enemy on any round where the enemy you focus on will not die before their turn and it is always better when the enemy you would focus on will probably die before their turn whether or not you target them or someone else.
I would say with sap it is easily half the time you are fighting multiple enemies. That is my opinion from extensive experience using sap. Sap is my prefered mastery, it is the one I have used most often (with nick being second most).
Magic items can effect everyone equally or close the ac gaps just as often as they expand them.
Yes they can, but they generally aren't. A character building for a high AC is going to prioritize items that boost that, particularly when they use attunement (as most that boost AC do).
For example I am wielding a +1 shield, that isn't a bonus someone who does not use a shield is going to get. I also have a ring and a cloak sucking up 2 of my 3 attunement slots (Wand of Fear is in the third slot). Other party members took the Wand of Paralyzation, Ring of Invisibility, Brooch of Shielding, Boots of Striding and Springing, Crystal Ball and Ring of Feather Falling we found (and probably other attunement stuff I can't remember).
Only if your ac is significantly higher than enemy attacks (but not too high) which for almost every 5e pc is not the case. And really if your being hit 5% of the time or something silly like that, why bother with sap to begin with.
Because with sap you are hit like 1 time in 400 if you are only hit on a 20, moreover sap affects whoever they target, not just you.