How does Henry Higgins get 'some of his own back' in Acts IV and V? When he says,"How the devil do I know what's to become of you?", he caused Eliza to feel unwanted and abandoned. He used his rough tone of voice and unpleasant words to show Eliza that he did not care for her. His exasperated and irritated tone expressed that he wanted to wash his hands off the whole affair. The quotation,"I know you don't care. You don't care if I was dead. I am nothing to you -- not as much as them slippers." has shown that he had given Eliza the impression that he did not care. The next quotation,"Eliza hopeless and crushed", has proven that he had hurt Eliza.
Mr. Higgins had used these words to hurt Eliza because at that point of time, Eliza had grown fond of him and loved him in a friendly way.
Therefore, she expected him to reciprocate her love and thus was deeply disappointed and hurt when he answered in that manner.
In Act IV, Henry Higgins also got a little of his own back when he was sarcastic to Eliza. He used the phrase "You may take the whole damned houseful if you like. Will that satisfy you?". Henry Higgins said this to Eliza when Eliza asked him what she might take if she wanted leave. She said that she did not want to be accused of stealing.
But Henry Higgins was very sarcastic, making Eliza appear calculating as she wanted to make sure of what she could take along with her. He was just trying to make her feel bad too.
In Act V, Higgins tried to shake Eliza's composure with the phrase "You will jolly well see whether she has an idea that I Haven't...