Great question! I'll begin with the Spartans themselves:
SPARTAN IIs were given:
-New bone material composed of carbon and ceramic, making bones almost unbreakable. No major side effects observed.
-Drugs that increase the body's production of enzymes responsible for the creation of muscle tissue, causing muscles to be larger, more dense, and more tightly woven. If you don't get the scientific jargon, this drug makes muscles really powerful. Side effects include Parkinson's and Lou Gherring's disease and cancerous growth in muscle, all of which were observed in multiple Spartan II canidates.
-Drug to enlarge the retinal fluid in the eye, allowing for perfect vision, even at night. No serious side effects reported. All Spartan canidates given this drug showed a change in eye color, often to abnormal colors.
-Drugs to increase the speed of reflexes. Known to cause temporarily and sometimes permanent blindness, excessive shaking, and shutdown of certain parts in the nervous system.
As a result, Spartan II's are faster both physically and reactivly, have incredible eyesight even in the dark, can lift almost three times their body weight, have strange eye colors, repressed sex drives, unbreakable bones, and can take a lot of damage that would otherwise kill another human, and allowing them to wear MJOLNIR Armor. However, all these enhancements did cause thirty of the original 75 SPARTANs did not survive the augumentations, and twelve were permanently crippled.
Spartan III's differ:
Spartan III's were given these enhancements:
-The same bone material given to Spartan II's.
-A degree of the reflex and muscle enhancing drugs, but not as much as Spartan IIs to avoid death or other side effects.
-The same eyesight drug as given to SPARTAN II's.
*The Office of Naval Intelligence found how to pinpoint exactly what causes some people to react negatively to the SPARTAN enhancements. Therefore, the Spartan III's with the correct genetic makeup, such as NOBLE Team, were given the same enhancements as the Spartan II's, giving them the same traits.
Spartan III's did not have the same level of performance as Spartan II's, and could not use MJOLNIR Armor. However, they were cheap and easy to mass-produce, and had a performance record equal to that of any ODST Unit. Unfortunatly, Spartan III's were considered "expendable" and were often sent on suicide missions to difficult for traditional ODST's and too risky for the expensive Spartan II's. Spartan III's had the highest mortality rate of all three Spartan programs.
No current fiction describes the exact procedures that Spartan IV's were given, however, since they are capable of wearing MJOLNIR Armor, the enhancements must be similar to the Spartan II's enhancements.
MJOLNIR Gen1 and Gen2 differ in a few areas:
-Gen1 has the layer containing the hydrostatic gel directly between the ceramic plating and the user's inside suit. The ceramic plating completely covers the suit. Gen2 has three layers instead of two. The first layer is obviously the part in direct contact with the user's skin. The second layer covers the entire body, and contains the hydrostatic gel. The third layer is consists of the ceramic plating, however the plating does not cover the entire second layer.