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So...
Knowing that AC is abstract in D&D, we really can't go with the realistic "if armor = X and weapon fired is from a pistol, ignore all damage, but if fired from a rifle caliber, then no damage is ignored, yada yada yada." That gets too complicated and is too much to remember. So my suggestions?
* No armor gives disadvantage to stealth. each plate is in an individual pocket and it doesn't make any more noise than your regular clothes.
* due to weight and encumbrance, I would give a max DEX bonus of +2 for level III or higher armor. Remember that level III and IV don't weigh that much different or carry different in any appreciative value. They are just made from different materials
* Level II armor gives DR2, Level III gives overall resistance, Level IV gives resistance or DR10, whichever is higher.
And be done with it.
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thanks for all this...a few things...now that the stimulationist pandora's box is being opened.
-The plating strength only tells about part of the armor. I assume that you can have more plates or less on different parts of your body? Probably easier to go to something that is more descriptive and distinguishes something that is easily concealed vs what a marine might wear vs something riot police in a post apocalyptic movie might wear.
-Speaking of plating...you can also (have a servant) oil your chain or plate mail. I would think the stealth penalty is coming from just trying to move carefully with a lot of bulk. Again, the coverage of the armor as well as the level might be a factor here.
-The level thing is interesting, but what this says to me is that scarcity (ie expense) is also a factor, something the article writer ignores. This would need to be worked back in, either through cost or some other mechanism (armor that combines kevlar and ceramics is like magic...)
-Then there is damage avoidance (DA) vs damage reduction (DR). You seem to endorse the damage reduction approach.
DR is often portrayed as more realistic then DA, but DA is easier, as it does not involve an additional math step.
DA
is easier. But in terms of realism, DR is good in someways, but doesn't really capture the case where a light weapon can bypass armor all together, a fact which is really relevant if you talking about partial modern armors. E.g. you attack someone with high DR using a dagger, or a .22 and may have no chance of overcoming the DR, or barely hurt them, when in practice you could hit them in the face, or somewhere else unarmored. On top of this, when you stop thinking about direct hits and more about grazes and hits at an angle, then a major thing armor will do is convert these to misses or close equivalents, even from higher caliber weapons. DA is a good approximation for this.
If you really want DR, you can deal with some of this through other mechanics, like critical hits or "exploding" damage die. Another approach is variable DR (d4 DR, d6 DR etc). I think a game designer named Mike Mearls came up with a version of that, about 10 years ago.
