When I said -3 or -4 I was referring to Dex bonuses in AD&D... so that pirate would turn out to be AC 6 or AC 5, which are the equivalent of AC 14 or AC 15 in 3E/4E.
???? Ok, I think conversing with both Erogaki and BryonD has got me all flummoxed


But then the 4e system isn't arbitary as you explain it. Tack on another 5 points for being a 10th level NPC and the AC in the 4e hits 19/20. Your example would ALSO apply to a PC.
Ok, what level was your PC as something is very weird here....A 10th level PC should be able to hit an AC of 21 without much trouble IMO.
Are we talking about the half-level mechanic or "how the NPC gets their numbers"?