If the +$1 is so much better, how comes my card tested perfectly fine at $4?
First of all, it's not "so" much better. I believe my exact words were "sliiiightly better."
Second, most of the issue with your card, IIRC, was the +Buy. +Buy often goes unused, and if it IS unused, then basically the functionality you got out of the card was way closer to balanced. When you really need that +Buy, however, it's a different story, albeit probably not overly conspicuously so even then.
Second, while playtesting is the most important tool you've got for card pricing, it can be super deceptive.
Here's a recent post about a card it took me, quite literally, somewhere around 20 games before I realized that my first, second, and third impressions were incorrect. For all I know I'm still wrong. The difference with your card, though, is that it's a pure vanilla card with a plenitude of other vanilla cards to make comparisons with, so one can be more confident.
Third, the wrong price on a card isn't necessarily going to break the game, or if it does sometimes it will do that in really subtle ways. Donald has said in the past that Throne Room works perfectly fine at $3, except that it became just a little too easy to stock up on them with extra buys. I believe he's said similar things about Village, too. Village is balanced at $2 -- in fact, it wouldn't break the game outright at $0 -- but the ease at which you can rush the stack with extra buys creates a less interesting game.
Where I'm going with this is that I think your card probably does
usually create a roughly balanced game at $3 or $4 -- that is, it isn't game-warpingly strong just because getting copies of them are more accessible in the early-game -- but in situations where, say, rushing the pile is profitable (e.g., when Villages are plentiful) and in particular when you can use all the abilities of the card including its +Buy, then you may find you have a problem. Because price DOES have ramifications in terms of (1) how fast can you build up an engine, and (2) functionality relative to the other vanilla cards in the game.
These two points are related, because the right combination of vanilla bonuses leads to an engine, and if your vanilla card doesn't fit with the price settings of the other vanilla cards, then you've got a gameplay imbalance relative to the other vanilla cards. Of which there are plenty, all basically flawlessly costed in sync with one another.
Let's say you had a fan card called "Blacksmith." It gives "+3 Cards" and that's it. It costs $3. Is it broken? Probably not, actually. It's obviously costed wrong, but on a great many boards it'll be perfectly fine. On some, though, you'll notice it's just a little too easy to get that drawing engine up and running, and/or you'll notice it's just so much better than other $3 cards that you'll be buying it above other $3 cards way more often than its probably ideal for the game. It'll still come down to a judgment call. But that's not to say that one judgment isn't better than another.