I'm not saying that 4th ed would be a good fit for the Dresdenverse so much as I'm saying that Harry falls into the rough pattern of how 4th ed works wizard types. Trying to cram, say, the Alphas into that pattern would be really contrived and wouldn't work very well at all.
As to the holiness of Michael and his armour, it's my impression that it's Michael himself that is holy. Faith of any sort seems to have power in the Dresdenverse, and Michael's faith is a monolithic and impressive thing. Harry has faith enough in his magic as to be able to change the world with a thought, but even that seems to pale before Michael's faith. While Harry can keep a Red Court vampire at bay by brandishing his pentacle, Michael empowers crosses on his person to be actively dangerous to them.