There are spartan 4's, or there soon will be. In the novel, Halo: Glasslands, Towards the end, admiral Protasnki (i know, spelled wrong) says that they will begin a new spartan program. but this program would be similar to the Orion project, or Spartan I program. they will enlist adults, not kids. There will be no augmentation, just steroids and stimulants. THey will just be super soldiers, without Mjonir armor or metal plates bones. The spartan 4's will just wind up like Sgt. Avery Johnson, a spartan I.