I think is a fine cost for this as the effect in most circumstances does nothing. There are decks like burn that just fold to it - especially if it represented in an enchantment form as decks would have a much easier time removing an artifact. In my recent set, I thought of even doing this at common, which admittedly is a bit much: Imperial Magus. The card doesn't explicitly say hexproof since I've also though of removing evergreen status from that keyword, but that's another topic.
Could be cheaper. Compare Witchbane Orb.
nice
Or would this be more stable at 4?
I'm not sure what you mean by "more stable".
I think Ivory Mask was the 'original' concept. Since then we have gotten Leyline of Sanctity, Imperial Mask, and Orbs of Warding.
On the creature department we have True Believer, Aegis of the Gods, and Spirit of the Hearth.
I think
is a fine cost for this as the effect in most circumstances does nothing. There are decks like burn that just fold to it - especially if it represented in an enchantment form as 
decks would have a much easier time removing an artifact. In my recent set, I thought of even doing this at common, which admittedly is a bit much: Imperial Magus. The card doesn't explicitly say hexproof since I've also though of removing evergreen status from that keyword, but that's another topic.
By stable I just meant safer, or more appropriate.
Don't forget Shalai, Voice of Plenty :)
Yeah this ability at common might be just too useful too often. I could see it at uncommon though.
it should say "you have hexproof"
Okay