Saying their AMO is 4" shorter is like saying their yardstick is 4 feet long. It it's 4 feet long, it isn't a yardstick, and if the string that fits is 4" shorter, then the bow isn't marked according to AMO. The string that fits the bow determines the AMO bow length, which is 3" longer than the correct length string.
AMO doesn't change from one manufacturer to the next, but it appears a lot of them don't know what AMO really is, which just adds to the confusion. It's really not that difficult. I have no idea why they won't label bows correctly if they are going to mark the bow "AMO". At least put "ASL" for actual string length on it somewhere. It's not like that would add more than a few seconds and a couple of cents worth of ink to the process.
How much difference in brace an inch of string length will make depends on the bow length and design.
I've gone round and round with a few bowyers trying to get a clue as to why something so simple, that would benefit all involved, is rejected. Got a few entertaining excuses, nothing that made any real sense though. Drives me nuts. It's not for my benefit, but theirs, but they act like I'm trying to take money out of their pockets. Go figure.