Avengers 4: 10 Theories From The Endgame Trailer

One of the most emotional scenes in the trailer sees Captain America and Black Widow together after having formulated some sort of plan and saying a farewell of sorts to one another. This is the scene where Cap responds to Natasha's encouragement that the plan will work with;

"I know it is. Because I don't what I'm going to do if it doesn't."

This feels distinctly like the team parting ways, as Cap leads one leg of the team on one journey, while Black Widow takes the rest of the team elsewhere. For instance, if Ant-Man and Cap do go time-traveling, Black Widow is staying behind with the other Avengers, embarking on a journey in the present day.

The only time we see Cap in the trailer is while he's either at Avengers HQ or on a Quinjet. Meanwhile, we see Black Widow off in Japan, looking for Clint. It's always been a safe bet that Avengers 4 would feature multiple threads of different teams of Avengers, much like Infinity War did. But this trailer seems to explicitly indicate that the team will be splitting into two groups.