Yeah, the reason the PHB doesn't include options for non-masterwork armor above +3 enhancement bonus is because it would mildly screw up game balance, tilting AC in favor of light armor wearers.
This is the kind of thing that's most appropriate as a house rule I think.
WotC probably should have included a suggestion box in the armor section that something like:
"At your option you may choose to include non-masterwork armors of +4 or better, though keep in mind it may have unintended consequences for game balance as you are changing the ratio between attack and defense across the board at later levels, and greatly reducing the benefit of heavy armor."
This is the kind of thing that's most appropriate as a house rule I think.
WotC probably should have included a suggestion box in the armor section that something like:
"At your option you may choose to include non-masterwork armors of +4 or better, though keep in mind it may have unintended consequences for game balance as you are changing the ratio between attack and defense across the board at later levels, and greatly reducing the benefit of heavy armor."