There were two post credit scenes shown in which one followed Thena, Druig and Makkari and the other featued Dane Whitman.
Firstly, following from their departure from Earth, Thena, Druig and Makkari set out to find other existing Celestials in which we see Pip the Troll making an entrance as well as Eros aka Starfox (who is Thanos' brother) played by Dunkirk's Harry Styles.
The second post credit scene follows Dane Whitman who is viewing at an ancient mystical swrod after he told Sersi that his lineage is rather "problematic" before she was taken in by Arishem. A voice appeared asking Dane "Sure are you ready to wield that" in reference to the sword. That voice is confirmed to belong to
Mahershala Ali who is portraying Blade within the Marvel Cinematic Universe.
After the scene, the message "The Eternals will return" appears and I am very interested to see how the future of characters will be incorporated within Phase 4.