If a recipe calls for 4 cups of mushrooms, chopped, assuming that I chop and then measure (and not the measure and then chop)?

I read it as measure then chop. Otherwise, it would read 4 cups of chopped mushrooms. Mushrooms are very forgiving in recipes, so either will be fine.

Interesting, and confusing. The way the sentence is formatted I would measure then chop. If the recipe said "4 cups of chopped mushrooms" I'd chop then measure. This is a good argument by using weights rather than volumes for recipes!

Unless the recipe tells you otherwise, You should chop first and then measure.

I would always chop and then measure - 4 C of whole mushrooms would be a big difference from 4C of chopped mushrooms.