Herald better than Ironmonger? That's Blasphemie in my eyes
Ironmonger is #1 on my personal $4 card list without any doubt.
Ironmonger is the only $4 card that that i can't remember
ever not having bought in a game it was present.
The only other card that may compete that is JoaT but i'm sure Ironmonger still wins.
Herald for sure has the bigger impact on boards you want it, but you always want ironmonger and often don't want herald(e.g. you usually don't want herald without any trashing, or when junking is stronger than trashing).
And the same is to say about the $4 Cursers, there are just so many good trashers in Dominion
that you can often ignore them.
And like Robz said i would even go Ironmonger-(X)-BM over most terminal draw-BM strategies
where X can be another decent Terminal but can slo be left out.
I mean early on it's a Peddler + sifting or even as powerful as an activated menagery.
And once you started heavy greening Ironmonger decks really dont stall very fast at all.