There is a unique aspect to the StarTropics game for NES. When you reach a certain place in the game in Chapter 4, you are required by your onboard robot to enter a radio frequency. This is not a puzzle, and there is no way to determine the correct code from within the game. Instead, the actual answer comes from the game instruction book. Perhaps some type of attempt at discouraging rental of the game, or as a method of copy protection.
Unfortunately, if you have the cartridge on its own, or you are lucky enough to own an NES Classic, you will have no way of moving ahead past this point in the game unless you want to spend a few hours testing up to 1000 combinations. I remember renting this game when I was a kid, and having to give up because I didn’t have the manual.
Luckily for us, the internet now gives us access to almost any piece of information we want, and this includes the frequency for StarTropics. You need look no further. Enter 747 and you’ll be on your way.
If you’ve made it to Chapter 4, you’re doing well. Be warned though, the game becomes even more difficult, with many surprises. Enjoy!