Also, Mab is queen of air and darkness, so Air would seem to be the appropriate element for winter.
They are under the two-party system, at least. If I was expanding it to four courts, I would snip an element from Winter and Summer each like so:
Seelie: Light (Growth, Wildness, Renewal, Fire, Light)
Unseelie: Darkness (Death, Wildness, Slumber, Ice, Darkness)
Abseelie: Air (Entropy, Wildness, Slumber, Frost, Air)
Adseelie: Earth (Birth, Wildness, Renewal, Warmth, Earth)
This gives each court a unique element (and you could make a light/dark/air/earth evocation system for a fae-themed but not Sponsored caster) and a unique effect - only Spring can create, only Winter can destroy, only Autumn can decay, only Summer can grow. Summer and Spring can both warm a room, but you'd have an easier time warming a person with Spring and an easier time throwing a fireball with Summer.