For a short answer to your question, op: I don't believe anything is automatically masterwork (of dark iron/dwarven), etc once you hit +4, or +6. There's nothing in the books that say when you have constructed a +6 enchant suit, that the suit is automatically godplate for a further bonus due to being masterwork.
I've been developing a campaign for 4E lately and come across this same issue. there are +1 to +3 magic enhancement available for armor. The only way these kinds of enhancements could be used is if they did not come on "masterwork" armor.
There is nothing in the Rules as Written or the Spirit of the Rules which says that once you hit +4 enchantments, you MUST use them with masterwork gear, which involves special resources, such as wyrmscale, etc.
The AC difference of a regular chainmail with Curseforged compared to Forgemail with Curseforged is 10 vs 13.
Curseforged also has +1, +2, and +3 values which require regular chain. There is nothing in the rules that say you absolutely cannot put a +6 enchant on a regular piece of chain. In fact, you could make the argument that you could do any number of tricks to balance out the game to your liking. Suppose you don't like the way cloth and leather users scale past plate users. Maybe your campaign has exceedingly rare events of getting masterwork cloth armor. I assume that's a rarer skill at any rate, since it's less likely to be as practiced as slaving at the altar of hammering and tongs.
While lots of the game design can't be changed without become house-ruled, you can feast or famine the party with drops all you like, to make sure it plays out the way you want it to. In fact, I think having a bunch of amazing twinky magic gear is what turns people off from 4E. You could sell regular non-masterwork chain with a +4 enchant for half price. The price is, in fact, set up in the book, but the price in the books is, I think, one of the things you can most easily manipulate without breaking into "house ruled" scenarios. It's not like changing the rules on pushing, or rolls for intimidation working 100% of the time, or some other business like that.
As a fan of trying to push the limits of the game while staying within a game's particular rules, there's a myriad of ways to control the game so that nothing is too easy or too hard. If your adventurers only have +4 regular chainmail, then mobbing with minions is still viable, and a way to spice up the game. Once you give the adventurers masterwork +4, the "mob of minions" might be replaced from regular minions to true enemies with hp.
There is no problem in 4E or any of the other games that can't be fixed by a DM. For me, if the party members are struggling with encounters, either I want that by design, or there's a small issue I can fix by throwing down free regular ol' chain/plate with a +4 bonus from a fight, and once they get back to town, they could upgrade to a masterwork version by paying with gold for a shopkeeper to make it for them.
There's a bit of an assumption that everything should scale perfectly, that everything needs to "work out ideally", but I think that is exactly what makes people feel like D&D 4E is more like WoW than anything else. It is through struggles and imperfections that D&D starts to feel alive and has a lasting impact. If it runs like a swiss watch that the players of characters are just riding a rollercoaster game on paper, then where's the fun?