I would agree with you on the fourth energy needing to be related to a form of decay but it needs a personal attribute that is tied to it like Solar does burn damage, Arc does chain damage, and Void drains and suppresses. If it was Cyro it be a bit to similar to Void since it already causes slow and disorienting effects.
I also don't think it should do burn damage like Solar so to make it have it's own effects I think it should cause confusion in your enemies.
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Decay with cold sounds more reasonable to me, with yours and the op's post explaining the logic. I was at first thinking decay as poison, and it still could have some poison like elements, but cold fits quite well.
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Freezes like mei from overwatch
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Kinetic. Causes knockback and stuns/disorients