I'm not sure I believe it's really balanced. At +$3, I can see it: a terminal Gold would be about a $5 card, and losing a card next turn is usually going to be a pretty small penalty, because chances are at least one out of your next five cards is a VP/Curse card, or, failing that, a dead Sea Hag or a clashing terminal or whatever. (Another way to look at it is that it is not even as severe a penalty as half a Militia. Militia is a pretty decent attack, but at half the power you can discard, for example, just your one Estate instead of your one Estate AND a Copper.) And if the penalty does lose a useful card, maybe that's because your deck is so rich with great cards that you can draw back up anyway.
Sometimes the penalty will genuinely hurt, sure. But often enough and severe enough to compensate for a $4 +$3 action this turn? Maaaybe, but maybe not. Consider that Oasis uses a similar penalty (this turn rather than next turn) to drop the power level of a card from approximately the $4 tier to the $3 tier. That's a much smaller gap than dropping from approximately the $5 tier to the $4 tier.
But what we're talking about is not a $4 +$3 action but a $4 +$4 action, which, without the penalty, would be priced somewhere around $7. I just don't see having to discard a card knocking the cost down by 3.
That said, I like the core concept, which I guess is like a Tactician in reverse: sacrifice the next turn a little for beefing up the current turn. I'm sure the idea can be made to work.