My inclination is to be pretty laissez faire, partly because I believe in letting kids learn things on their own as much as possible even if that involves some bruises and scrapes, and partly because it's just easier to let 3 and 4 year olds do what they want. I really don't enjoy having any kind of confrontation with someone else's kid, so in the first place I try not to interfere unless it really seems necessary. If I do decide I need to stop them, I first try telling them to stop. This sometimes works, depending on the kid and situation, but often they just ignore it. At that point, I usually just give up and find an actual teacher, since they have no qualms about laying down the law when needed.
The interesting part is how different people's instincts are, and how differently they approach this question. I see some parents who handle things similarly to me, but there are others who are a lot more cautious and strict and don't hesitate to get involved.
The teachers advise volunteering parents to stop anything that makes them feel uncomfortable, and I think that's what is really tricky about all this. There's often not time to think things over so you have to decide in the moment based on your instincts. The teachers know that each parent will handle things differently, and they're ok with that. They would rather have a parent go with their gut and possibly overdo it than second-guess themselves, and I think that's the right approach. If I'm entrusting other adults to take care of my daughter, even if I know they're going to handle some things differently than I would, I'd prefer that they are acting decisively if something doesn't feel right to them.