Act IV
Summary
All of Act IV takes place in Hardcastle's house.
Hastings and Constance enter, bringing news that Charles Marlow  (father of our young hero) is expected to visit the house that evening.   Since he would surely recognize Hastings and thereby ruin the plan for  elopement, the lovers know they must move with speed.  Hastings has  meanwhile sent the casket with jewels to Marlow for safekeeping.  Before  she exits, Constance says to herself that she will delude her aunt  "with the old pretense of a violent passion for [Tony]" so as to keep  her off their trail.
Marlow enters with a servant, confused why Hastings sent him the casket.  He asks the servant to bring the casket to Mrs. Hardcastle  (whom he still believes is the landlady) for safekeeping (uh oh!) and  then speaks to himself about his nascent passion for the barmaid.  
Hastings  enters and Marlow tells him about the barmaid and his new infatuation.   Hastings is shocked that Marlow would rob a girl of her virtue, whereas  Marlow insists he will "pay" for the virtue.  When Hastings inquires  after the casket, he's angered to hear Marlow has sent it to the  landlady (since that has returned it to the hands of his antagonist Mrs.  Hardcastle).  However, Hastings cannot reveal the reasons for his  displeasure without alerting Marlow to the duplicity being played on  him, and so Hastings must decide on his own that he and Constance will  leave without the jewels.
Hardcastle enters to find Marlow, whom  he welcomes again as son to his old friend.  However, Hardcastle (who  Marlow still thinks the landlord) wishes Marlow to control Marlow's  servants, who are getting drunk and causing a ruckus.  When Jeremy,  one of the servants, enters drunkenly and makes a fool of himself,  Marlow refuses to discipline him but instead mocks Hardcastle's request.   Fed up, Hardcastle demands Marlow and his servants leave immediately.   Marlow is disgusted with the idea of being put out in the middle of the  night, but Hardcastle insists until Marlow asks for his bill.  In the  confusion over why Marlow is requesting a bill, Marlow suddenly realizes  what is going on, but not before Hardcastle exits angrily.
As  Marlow is grappling with his mistake, Kate (still disguised as barmaid)  passes through, and he confronts her immediately about where they are.   Realizing she needs to play the situation right so as not to counteract  her well-designed ruse, she answers him that it is Hardcastle's house,  and laughs at the prospect that he considered it an inn.  What's more,  she provides she is not a barmaid but a "poor relation" who relies on  the Hardcastles for the charity of shelter.  Marlow is shocked to have  potentially treated her as a lower class woman, and apologizes for  having mistaken her behavior for that of a barmaid.  He admits to her  that he cannot pursue her since "the difference of our birth…makes an  honourable connexion impossible" and so he must not endeavor to ruin  her.  Kate is impressed with the virtue he shows here, and she suggests  that they could be wed even if she lacks fortune.  He is touched by her  "pretty simplicity" but admits "I owe too much to the opinion of the  world, too much to the authority of a father," and so he leaves her as  an act of courage.  When he leaves, she decides to herself that she will  maintain the deceit long enough to show her father his true character.
Tony  and Constance enter, with the former explaining that his mother  believes the missing jewels were due simply to a servant's mistake but  that he cannot steal them again.  However, he has prepared some horses  for their escape, and if he and Constance can fool his mother for a  while longer, she and Hastings should be able to escape.  As Mrs.  Hardcastle enters, they pretend to be caught fondling each other, and  she, so happy to see it, promises she will have them married the next  day.
A servant brings a letter for Tony, the handwriting of which  Constance immediately recognizes as belonging to Hastings, which could  ruin them.  Tony, who cannot read, tries to sort it out, but before he  can give it to his mother to read, Constance grabs it and pretends to  read it, making up a nonsense letter on the spot.  Her attempts to blow  it off don't deter Tony, who gives it to his mother to read.  She reads  from it that Hastings awaits them in anticipation of the elopement.   Though polite, she insists she will not be bested at this game, and  decides she will use the horses Tony prepared to bring Constance far  away from Hastings and any attempt to run away.  She then leaves.
Constance,  now depressed, is joined by Hastings, who accuses Tony of betraying  them.  Before he can suitably defend himself, Marlow enters, angry at  having been duped.  In short order, everyone turns on Tony.   A servant  enters to inform Constance that Mrs. Hardcastle awaits her for a quick  departure.  In the meanwhile, the resentment between everyone grows  harsher.  With a quick and sad goodbye, Constance exits.  Tony suddenly  develops a plan, and tells everyone to meet him in two hours at the  "bottom of the garden" where he'll prove to all he's more good-natured  than they believe.