It could be argued that (4) isn't essential, since you can run your games on an actual Atari 2600 or 7800 if you have the right tools for doing that- e.g., a Supercharger, Cuttle Cart, Cuttle Cart 2, or Krokodile Cartridge, and anything else required to load the game into said devices. Well, all you really need (absolute essentials) to develop games for the Atari 2600 are (1) a computer, (2) a text editor, (3) a 6502 assembler, and (4) an Atari 2600 emulator.