Thanks for the feedback, everybody.
@Kahryl: I like your cost-reduction idea, but power level aside, I worry it might have tracking issues. If you're playing a bunch of Inventors every turn, it would probably get very tricky to remember which cards have a reduced cost this turn and by how much.
@Dsell: I've been playing a few test games with it and it's tough to say whether it's too weak. In a game where you and your opponent are going after different strategies, it certainly whiffs quite often (50% or so). On the other hand, if you whiff it's probably because you just flipped one of the cards they need for their strategy. But once you start taking that into account, you start getting into targeted Attack territory, which I'd like to avoid. Hmm…
It does really suck when you flip an opponent's Tunnel and don't have any Tunnels yourself, I'll say that.
@blueblimp: I suppose you're right about it being a bit vanilla. I'd hoped the unreliable nature of the card would avoid that issue. In my opinion, adding +1 Buy doesn't really help that issue. There's also the fact that I'm trying to fit this card into an existing set. The set already has enough Attack cards (3 in a 13-card set), so I can't do that.
No other card in the set draws more than 2 cards, so I need this one to have that potential. +4 cards by itself might be balanced at $5, but I'm guessing it's too strong and it's certainly too vanilla. I am definitely open to ideas, though.