... It would be the only opening 4-cost move I can think of that would be extraordinarily likely to get you 6 for a Gold, if not more, on your first draw of it. The only way it wouldn't that I can see is if you ended up (in your resulting 6-card hand) with all 3 estates (or your second action, if you were silly enough to do that instead of a Silver) and no Silver. That feels, instinctively, better than straight +3 cards, but I haven't run the math. Certainly feels better than any $4 opener we currently have...
Seems like it would be crazy powerful in the early game, and only somewhat weaker late game *if* you manage to push your avg. value above $2.
Some of this reaction is based on my wife really not liking attacks, so we tried some home-brewed variants that tried to replace the "attack" with what felt like the opposite "benefit" to you instead. A witch that gave +2 cards and let you trash a(or a few) cards, etc. We tried Militia at first as +2 cards, +$2, and it was really dominant. Even +$2, +1 card was solid, though at least it wasn't head and shoulders ridiculous. Of curse, that was Base set only and we were sorta-noobs back then.