Instead of making it harder to hit a well armored target, it should be easier (slower to move). But your damage should be reduced, not your chance to hit. And then of course there is the penetration value of some weapons v. some armor types. Is there any simple conversion out there that does not need an added stat for weapons and armor to handle this? I am looking specifically for 4th edition D&D.
Yes, if you're willing to do some statistics
There is no method that I know of, currently, but it is quite simple to perform the necessary calculations based on expected and averaged damage. This formula is copy-pastd from my post here.
This formula indicates average player DPR against an equal level monster. To give monsters damage resistance at the cost of AC, simply decrease the "14" in the equation to a value you deem useful, take the difference between the old and new formulas, and provide that as DR that isn't "Resist" (otherwise resist-piercing weapons become overpowered).
Importantly, here, DPR is a function of accuracy. It is somewhat important to preserve the average damage numbers on both sides and I strongly dis-recommend changing non AC defenses: the riders on auto-hitting controller effects that nominally require accuracy doesn't bare thinking about.
However, I personally don't recommend this. Instead, and to represent this, vary your description based on how close the hit was to the monster's AC. For very low rolls, you wiff. For close to the number, you "clang" off the armor. This produces the exact same effect as the statistical fiddling above, without any messy math. 4e is not a game where you want to simulate "armor" as "damage resistance" as that creates a cascade of changes and balance tweaks that could quite easily break the game.