I arrive at JFK on a Delta flight in terminal 4 at 3:00 pm. I need to catch a Lufthansa flight in terminal 1 at 6:00 pm. Do I have enough time?
You certainly have enough time and nothing to worry about if the flight legs are on the same ticket. In case you bought separate tickets to JFK and from JFK, then you'll still probably make it, but in case the first flight is 2+ hours late, then you're very likely to miss the second flight and you're not entitled to a compensation from the airline. In case you have a good travel insurance covering those cases, you might want to take the risk.