Le 4e mur takes the secret drinking spot to a new level. It's modeled after the speakeasies which became popular during prohibition and it does that very well. In summer they also have a terrasse which is hidden from the street and feels very secluded. They make some excellent cocktails which may seem expensive but they are strong so you get value for your dollar. There is a small food menu but make plans to eat elsewhere because it's mostly snack food so it won't fill you up.
Getting the address is very easy. All you have to do is visit their website and register. They will then proceed to send you the address and instructions for how to enter. The only question is will you figure out how to enter the bar?