In Digital Communication, is there a way to add a fourth image or text block in a single row, or is three blocks full capacity?

It is possible to add a fourth, however it is not recommended as it can cause issues with cell width and email rendering. To add the fourth block, maneuver around in the Editor to get all four in the same row.

Best practice is once you insert 3 of the cells, nest the fourth below one of those three and then move it into the same row. Insert these cells before any other content in the email, makes the process easier.