After seeing this comment, I read this answer and this answer.
This gave me the distinct impression that any magic armor with an enhancement bonus of +4 or more is automatically masterwork armor. I found this rather surprising.
The answer to the second question mentions a math fix in regards to AC. My question is focusing on an armor's material. Specifically, I'm trying to determine if the rules as written (and as intended) require that any magic armor over a certain enhancement bonus be of a certain material. Furthermore, I'm curious about the assertion that enchanting a weapon would change its construction material. Obviously, once could just wave one's hands and say that the material of the armor is whatever, but I can't consider that an answer. If I'm houseruling things, I would at least like to know that I'm houseruling things.
I then looked back over the masterwork armor rules in the Player's Handbook, and found that masterwork armor has to be magical, which I was not aware of. However, I found nothing to imply that magic armor would automatically be masterwork, beside a statement that "the cost of masterwork armor is included in the cost of magic armor." That statement simply means that you don't have to pay extra for the armor to be masterwork, not that the magic armor is masterwork.
I then looked in the Rules Compendium, and on page 281 I found a statement directly contradicting magic armor always being masterwork:
...so a set of +5 black iron plate adds a total of 13 to the wearer's Armor Class (8 from the plate armor and 5 from the enhancement bonus)
If the +5 plate armor was masterwork, it would add 11, not 8.
Wax Eagle has pointed out tables in Heroes of the Forgotten Lands and Heroes of the Fallen Kingdoms for magic armor which specify AC bonuses based on enhancement bonus. These do not match the masterwork armor bonuses, and nothing in that table mentions construction materials.
So, is magic armor actually automatically masterwork, or are people simply assuming that if you're going to buy magic armor that is eligible to be masterwork that you will opt to get it as such?
If it's just an assumption about rational economic behavior, then enchanting mundane armor with a bonus over +4, or increasing the bonus on an existing magic armor past +4, wouldn't cause the armor to become masterwork. It would just add the enhancement bonus. Right?
If so, then this would contradict a statement from the second referenced answer:
If you're allowing players to upgrade the enchantments on a piece of armor, the armor should automatically improve to match the appropriate masterwork quality.