4 things to clarify.
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The original spartans were actually the Orion project super soldiers retroactively dubbed spartan 1’s. These individuals were much like the 4’s selected because they showed a notable level of skill and personality traits compare to others in their field of expertise or military division.
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While spartan 2’s have had years of military training and experience with much more rigorous augmentations compared to the 4’s, it should be noted that the 4’s aren’t exactly combat novices either. All members of the spartan 4 program were carefully selected from the best of the best of the UNSC armed forces be it army, marines, air force, navy, oni and their respective special forces divisions. The majority of those soldiers are active combat veterans with distinguished records and several years of experience under their belts; needless to say they are not average run of the mill military grunts in power armor.
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Mjolnir Generation 2 was specifically tailored for use by spartan IV super soldiers much like Mjolnir Gen 1 was tailored for the needs of spartan II’s and SPI for spartan III’s. Each spartan IV receive augmentation sets tailored for their genetic profiles and their Mjolnir Generation 2 armor is constructed to complement their individual augmentation profile. In particular I think the biggest reason why Mjolnir can amplify the strength of a spartan IV to greater heights than a spartan II comes down to one key augmentation- A specialized muscular coating which enables their musculature to work significantly harder without destroying their enhanced skeletal structure.
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While it is true that spartan 2’s are superior compared to 3’s and 4’s (Whose strength have been noted by Buck to be within the range of the spartan 3’s strength capabilities), All three classes are still weaker than most of the species in the halo universe. Beings such as Hunters, Brutes, and Drones can all but overpower a spartan in a contest of strength even in Mjolnir. But even some elite warriors ranging from Majors to Zealots are more than capable of fighting and besting spartans in combat. Hell Thel Lodamee single handily bested and killed Spartan 2 Sheila-065, he also bested John-117 the master chief and would have killed him were it not for the intervention of supreme commander Luro Taralumee- goes to show that no matter how experience and well trained a soldier is they can be bested and killed by those more naturally skilled.