[4.16 - Crossings]Bjorn's fleet sails onward and launches a surprise attack; back in Kattegat, Lagertha continues in her quest for power.
[4.17 - The Great Army]The Viking army begins to gather in Kattegat; Lagertha must remain vigilant in the absence of her son, Bjorn.
[4.18 - Revenge]Ragnar's plan comes to fruition; the Viking army lands on the coast of Northumbria.
[4.19 - On the Eve]The Viking army causes panic in the English countryside as King Ecbert and Aethelwulf plan the defense of the realm.
[4.20 - The Reckoning]Prince Aethelwulf finds himself subject to the Vikings' battle master plan; Ecbert remains behind with a plan of his own.
Source: pic + descriptions
These are definitely spoilery, so... proceed with caution lol