I'm generally of the belief that trashing is just about always right, except maybe for Mountebank, but the four-copper situation is one I often wonder about as well. I usually end up trashing, but I'm not sure at all that I'm right.
I think that it would be valuable for the community to do some serious thinking about a framework for evaluating trashing a card. We all know it's great, but how great is it? When does it stop being so great? How does it depend on what else is available?
I started to try to do some analysis of the four-copper situation, but it's *really* complicated. I think the main concern though should be: "Do I have time to build up my economy, or is there a card that I need RIGHT NOW to be competitive?" For instance, if there's going to be a minion fight, I might go for the Gold, since that will help me get to Minion money very consistently. If instead I'm going for a +actions +cards engine, I might trash, since that will make finding my engine cards much more consistent. Also, if it were going to be a Conspirator game, I would definitely trash, since I wouldn't even really want gold in my deck that much at all.