Well, since there isn't a "Max Enhancement Bonus" stat for the various armors, the way I read it is this:
Plate (+8 armor bonus) can have an enhancement bonus of +1 to +6, for a total bonus to AC of +9 to +14.
Warplate (+11 armor bonus) must have an enhancement bonus of +4 to +6, for a total bonus to AC of +15 to +17.
Godplate (+14 armor bonus) must have an enhancement bonus of +6, for a total bonus to AC of +20.
The materials to create plate, warplate, and godplate are very different - you don't just get warplate for upgrading your plate to a +4 bonus, you'd have to make an entire set of warplate, and then enchant it to a minimum of +4. Each masterwork armor's particular method of manufacture is in the description for the base armor type. That method is what makes it special. For example, "Feyleather armor is cured by an elven method that leaves the armor supple but tougher than normal leather. Starleather armor is infused with the raw spiritual matter of the astral Sea, making it light and strong." (PHB p. 212)
Now, a DM could of course choose how they want to deal with it. Some DMs might rule that normal plate, as part of the enchanting process, also becomes warplate. I personally wouldn't allow a player upgrading "normal plate" from +3 to +4 suddenly have the "normal plate" upgrade to warplate as well. Dwarves didn't make it originally (probably), and it certainly wasn't made with the same techniques used to make warplate (or it would've been warplate in the first place).
So, the way I see it, a suit of armor is first made (which determines it's quality) and then enchanted. Some armors must be enchanted due to the method of their manufacture.