The SPARTAN-IV Program was initiated by Admiral Margaret Parangosky after the Human-Covenant War. Instead of kidnapping young children to serve as SPARTANs, the IVs are volunteers from different branches of the military. Some are stationed near the UNSC Infinity in the Oort Cloud. Similar to the original ORION Project, the SPARTAN-IV Program would only consist of adult volunteers, as opposed to the young children. When the Program was installed, the remaining SPARTAN-III's were offered to be integrated into the SPARTAN-IV Program. The first active SPARTAN-IVs were already active by 2553, as some of them were temporarily assigned to the UNSC Infinity before it was scheduled to launch.
UNSC shouldve made spartan IVs in the first place. the war with the covenant wouldve ended much faster.
In reply to Ar7emis, #4:Have you read Glasslands? The war didn't completely end with Halo 3.
Parangosky did know about the clones eventually, but she reveals its not untill years later in the project.