As act IV begins, Antony is the emperor of Rome true or false

The answer is False.

In the beginning of Act IV, Antonius, Lepidus and Octavius were preparing to kill Caesar. They have prepared an army to kill Caesar. They had asked Cassius and Brutus to join them in plotting the murder of Caesar. He did not became King in this part of the play. The whole part of Act IV revolves on the plot of killing Caesar.