I'm flying from Colombia to Munich with transfer in Atlanta. The transit time is 3h and 58m. The flight is on Sunday and Atlanta, indeed, is the airport with the heaviest passenger transit in the world. Is it reasonable to think that I will have sufficient time to change flights? Am I in high risk of losing the flight? Bear in mind that I will have to pass immigration with a tourist Visa (last time at JFK, I spent almost 2 hours doing the immigration row). I don't know the airport or if I will have to change terminals.
Atlanta is a very efficient airport.
They have a large immigration and customs area, which moves passengers through expeditiously. If your bags are checked through to Munich, then you simply drop that at recheck just outside customs, clear security and head to your gate.
If you are flying on different tickets or airlines, then you may have to recheck your bags. The International check in is just outside F concourse, so doesn't take too long to reach it for check in.
All concourses are interconnected by a train which runs every five minutes or so. But most International flights depart from E & F concourses which is where you will likely arrive.
4 hours should be ample plenty.