The opening sequence from Episode 1 of Book 4, "After All These Years".
IGN also provides a synopsis for the season:
"In Book Three, Avatar Korra had to save the world and endured her most intense battle yet. Now broken and bruised, she begins a journey of self-discovery and survival in a new world where the strong Prey Upon the weak. When an aggressive military force rises up led by the power hungry Kuvira, the fate of the Earth Kingdom hangs in the balance and only the Avatar can set things right."