Compare Othello's language in Act 4, Scene 1 with Iago's language in Acts 1-3. What is implied by this in Othello?
It is at this point that Othello's language has begun to fall from what we first saw as he met with the important men of Venice. Though he said he was not blessed with the "phrase of peace," he was very smooth-tongued. By Act IV, this smooth tongue is gone and he is rough and violent in his language, particularly when he talks about killing Desdemona, saying at one point that he ought to "chop her into messes," among other gruesome things.
Iago previously had used some of the more base images of the play to try and get Othello's mind to the point where he would be willing to kill Desdemona. This shift in Othello's signifies the success of Iago's plan and his earlier crafty use of language to get to this point.