When a target is prone do you give the attacker a +4 to hit or give the target a -4 to AC?
When a target has cover do you give the attacker a -4 to hit or give the target a +4 to AC?
Mechanically they work out the same. The only difference is who does the math.
If it says "+4 to attack" it's +4 to attack. If it says "-4 to AC", it's -4 to AC. Cover is -4 attack, prone is +-4 to AC.
You are right the only thing I could think of is when their is multiple enemies -4 to a targets ac would apply to everyone attacking him while +4 to your attack only effects you.
It does matter if someone is using the parry portion of Opportune Parry and Riposte as a Swashbuckler. In that case, you are comparing attack roll to attack roll, so AC isn't involved in the parry. On the riposte portion though, it works like attack vs. AC...
I tried to think of other weird attack vs. AC things, and even on the oddball hit-by-5 mechanic of Mirror Image, I don't see that it matters.
I cannot think of any other mechanics that rely on a modified roll for attack where there would be a fixed target die number. If critical hits allowed using the modified roll to get into the threat range, getting a +4 for a prone target would be a huge difference, but they do not [for good reasons, this among them].
The time it really matters is double dipping. I have played with a lot of people who will add +4 to hit vs. a prone target rather than what the rules say. So long as everyone is on the same page this is usually no big deal. But when the GM is also applying the AC penalty to that same target you run into problems.
Things that add to AC add to AC, things that add to attack bonus add to attack bonus.
This matters if, say, you don't advertise your monster's ACs. You can just note that a monster went into total defense and gets a +4 to AC rather than telling the players that they all get a +4 to hit this guy.
