Spartan-2s: A bunch of total badasses that can take on a covenant army with a toothpick and win. Very expensive, and with a high rate of deformities. Many came out sociopathic, but most of blue team was about as close to human as possible, they joked, blue team was devastated after Sam died. Very few survivors left.
Spartan-3s: easier to make Spartans with cheaper armor. Fell under army jurisdiction, instead of ONI. Most were sent on tactically vital suicide missions, but some were hand picked to bolster spartan-2 ops. Many Spartans died so that the UNSC would have enough time to prepare for major battles.
Spartan-4s: mostly UNSC ODSTs, they are experienced, and well trained, but not as mechanical and disciplined as the famous twos. They suffer from somewhat of a god complex, as before, they were already elite fighting badasses, but now, they can throw cars. Yeah. They come across as jerks, but when it hits the fan, they will do anything to succeed.