Forgive your enemies, but never forget their names. (quoted from J.F.K)
I think the four silhouettes are simply Power Rocketed Anti Nitro Geared Exo-suited Recruit Subjects.
What really happens is Master Chief goes back in time to Reach the Fall of the Forunners and saves everyone from the Onyx Shield in doing so activates the five silhouettes hidden powers into certain colors, i.e. Red, Blue, Black, Yellow, Pink. They acquire these powers and the covenant are at a defeat and in doing so send highly trained elites to come to the planet in battle with these five masked hero's. Every time they defeat these elite aliens they are required to summon 5 machines in order to defeat the now larger elite alien. A Warthog, a Pelican, Long Sword, Scorpion and a Hornet. These five machines combine into a Mega Zero Orbital Rocket Drone.
I think my theory is pretty damn accurate to the show as well as the few books they have released.
/discuss
[Edited on 10.24.2009 9:48 AM PDT]