The activation cost is a bit lower than that on Nivix, Aerie of the Firemind. But that was an unutterably awful card.

Mm, Nivix is 4-and-Tap to "maybe 'draw a card', but only until your next turn - then you have to get rid of it".

I often wondered if they forgot to add "without paying its mana cost".

Yeah, I mean, 4-and-Tap to draw a card is fine - Jayemdae Tome on a land would be too good. But 4-and-Tap to have a shot at drawing a card if it happens to be an instant? Along with the other restriction you mention, it seems so bad.

But this card may be too far the other way: it's rather easier to stuff a deck with artifacts than with instants/sorceries, and this doesn't have the exile if you miss. (Though if the only nonartifacts in your deck are basic lands, you might rather have the exile-if-you-miss.)

I'm a little concerned about whether this is too good in-set, particularly bearing in mind CH's earlier comments on Rooftop Garden. I'll give it some hefty playtesting and see if it feels too strong.

Actually, this may be most interesting if it's not itself an artifact. So it's still good in an artifact deck because it digs up artifact cards, but doesn't actually self-synergise like that. That seems a bit safer and saner.