Tapu Koko could be Summer due to Koko being a fast Pokémon and Summer being known for its large amount of activity and getting outside. Electricity is also able to create heat, so a Fire transition isn't too far fetched.
Tapu Koko - Fire/Fairy
Tapu Lele could possibly be Spring since that is a time where a lot of growth and bloom comes to mind as well as Tapu Lele stemming its power from flower aroma. Tapu Lele is also intended to resemble a butterfly totem and butterflies are attracted to flower pollen.
Tapu Lele - Grass/Fairy
Tapu Bulu could be Winter and be based on an oxen or mountain bull rather than a standard bull it already has as inspiration. Its lazy nature could also bring to mind the slowing of the cold Winter.
Tapu Bulu - Ice/Fairy
Tapu Fini could be Fall due to it living in fog, and though Fall isn't a particularly foggy season, it could fit with the possible Ghost type you would give it.
Tapu Fini - Ghost/Fairy