Is 4 hours between flights at Schipol sufficient to make a quick visit to downtown Amsterdam?

You will go through immigration or passport control where they stamp you into Schengen and then customs on the way out landside
I've been through AMS many times; I'd go for it; and then I would head back to the airport at 1:30.

edit : my bad I did not see that you have two connections what a drag; Tom_MN is correct you will go through immigration at CDG

Are both your flights on a single ticket, or two separate tickets?
If you are delayed getting back to the airport, and you miss your flight, what would be the consequences?

What would you do in that 40 minutes of wandering?

If you want to toke up I know a good coffee shop close to the train station from the airport

Twenty years ago, the Schipol brochure actually suggested that. These days, it is just too short a time. You will clear Schengen immigration in Paris, and customs, normally just walking out the "Nothing to declare" door at Munich. I'm thinking it will take 30 minutes to get off the plane and walk to the train station. Another 15 minutes from the time you locate the ticket window, buy tickets and head down to the platforms and wait for the next train, 20 minutes for the train ride to Centraal. So that is over an hour. Figuring that you should start back to the airport at least 2 hours before your scheduled departure, your time is barely enough to stick your head outside of Centraal and snap a few pics and head back. Do you have carry-ons you need to store in a locker? That will eat up some time.

I think that nowadays, 6 hours is what is required to make the effort to go into Amsterdam on a layover.

Four hours is not enough. I would not take my chances. I did it with having eight hours.

Too tight for comfortunfortunately.

Not worth to miss your connecting fligth!

Do you really mean changing planes twice (3 flights)?
1) Indianapolis - Paris
2) Paris - Amsterdam
3) Amsterdam - Munich

I would say no way do that routing. There absolutely must be plenty of flights from Paris (whichever airport) to Munich.

If you do this routing:
You will go through immigration (=passport check = check on people) at your first Schengen Area airport, that is Paris.
Your bags (not you) will go through customs whenever they are returned to you, presumably Munich. Customs is a random stop only. Most people are not stopped.

And I would not want to go into Amsterdam with only 4 hours between flights.

We usually have a 3.5+ layover at Schiphol. Never felt the need to leave the airport. We would rather be "bored" than miss our connection. Having up to 4 hours gives us downtime to exercise, read and have a snack.