The trailer is said to be coming on Wednesday, December 5th and that was backed up by a couple trusted entertainment reporters who include the following:
Ok now that it's out I Can tweet about it...
Avengers 4 trailer on Wednesday morning. We're so close. Just a little longer.
— Daniel R (@DanielRPK) December 3, 2018
Yeah all the rumblings seem to be true. A new Captain Marvel trailer coming Monday night, and hold on to your seats because Wednesday more Marvel goodness should be hitting.
— Peter Sciretta (@slashfilm) December 3, 2018
I’d say that it’s a safe bet that the trailer will debut on during Good Morning America on ABC at 6am Pt. That has not been confirmed,