I think this would need to be templated a bit like Painter's Servant:

> Until the end of the next turn, you, permanents you control, and cards you own that aren't on the battlefield all gain protection from the colour of your choice.

I'd agree that CMC 4 is a bit scary for this spell, since it's a counterspell of sorts as well as a mass-evasion and mass-combat trick spell. It's not any one effect that does it, but the flexibility and sum of all the possible uses of it that make it something to really watch out for. It might be ok at , but a card with such a flexible effect has a pretty good chance at getting broken somehow.

How does this card compare with Silence? I think that's a good reference point, but I'm totally lost on its potential power level. Am I hosing it too badly with those drawbacks?

If there's some way, ANY way, to recur this from exile... It's still only as bad a Platinum Angel. So it's probably fine. Maybe even downcost the additional cost, as it doesn't protect your creatures. Like Holy Day if you're losing the board position, this card will only stave off defeat for a turn or two. Worse; you need to hold the mana ready just in case your opponent pulls out a win fireball on their turn.

I don't think this needs the additional cost in multiples. It's similar to Angel's Grace.

Yeah, additional costs in multiples kind of confused me too. You don't want players putting four of a rare in their decks?

I'd never heard of Angel's Grace before, so I didn't realize how similar my design was to an existing card. From here I'll treat it as a better Angel's Grace.

It's REALLY hard to come up with effects that are sane, balanced and that haven't cropped up before somewhere :)