You can pick any 4 you like from your list. Heck you can even vote with your money on which would be in the top 4 (depending on how many align with your ideology). I am just speculating it will be 4 because of how all other sectors have at least 3 or 4 well-entrenched players. It seems to be by design. My guess is that it is driven by structure of the Military-Industrial Complex. That has at least 3 or 4 vendors in each sector, so as to not over-rely on any one company and create a monopoly in that specific industry.