Doug and Carrie feel that they are ready to become parents. Their only problem is that Arthur is always around, and never gives them any time to themselves. Doug hires a dog walker named Holly to walk Arthur for a couple hours. Arthur does not know that Holly is a dog walker. Carrie uses the time for her own personal pleasures. Doug hates that she is not paying enough attention to him with the time he is buying. Eventually, Arthur finds out what Holly does. Eventually, Arthur and Holly become good friends.
For Carrie's birthday, Doug decides to get her laser eye surgery. Doug brings her to the place, and the surgery is a success. The next day, Carrie wakes up to blurry vision. The next day she wakes up to the same thing. Doug calls up the doctor and asks if these symptoms are normal. The doctor says they are not normal, and Doug is now very nervous. Carrie finds out that Doug went to a cheap place, and used discounts.
Doug is ready to break the IPS record of most consecutive day without a broken package, or complaint. While he is ready to break it, his co-workers are not so excited. The old timers are not happy with Doug because he's going to break the record held by their old friend. Even Deacon is mad at Doug, because Doug broke one of Deacon's packages years ago. On the last day, Doug has to deliver light bulbs. He has to go up stairs to deliver them, but can not because the stairs are wet with paint. Doug has to throw the bulbs up or the company will call Doug's supervisor. It …
The friendship between Doug & Carrie and Deacon & Kelly is seriously tested when Doug smashes Deacon and Kelly's car.
Carrie brings Doug to her company retreat for the weekend. Carrie tries to get Doug to look like a successful person, but he puts on a bad show. Doug tells Carrie that he will try to work harder to her get a promotion. He even tells one of the senior partners that he climbed Mt. Everest.
On Halloween, Doug buys a pumpkin for the tricker treaters. Arthur comes in the kitchen and smashes it. Arthur has “banned” Halloween from the Heffernan household. Doug goes back to the shopping center to get a pumpkin and a whole bunch of other Halloween decorations. When Arthur sees this, he has a small heart problem. Arthur has become very angry at Doug. He tells Doug that when he was a child, his father made him trick or treat for hours just to get pennies. That is why Arthur hates Halloween. Doug makes it up to him by getting him a childhood sweet treat at a …
While Arthur is in the hospital getting Angioplasty, Carrie and Doug go through his room to find his sleep mask that he requested. Carrie finds an acceptance letter from Florida State University. She knows that he kept this letter from her, so he would have a better life. Before Arthur goes in for surgery, he tells Carrie that he never told her about a phone call from her boyfriend in high school. In the meantime Doug has become an emergency contact to a man he delivers to named F. Moynihan.
When Arthur comes home from his operation, Doug and Carrie decide to install a nanny-cam to watch over him. While getting a life insurance plan, Carrie decides that if she unexpectedly died, Arthur would continue to live with Doug. Doug really doesn't like the idea. In the end, Doug, Carrie, and Arthur find that being together is more important then finding out who gets stuck with who.
This episode is set before Doug and Carrie's marriage. Carrie is very nervous to be married to Doug. She can not stop vomiting. When the two meet with the priest, he remembers that Doug and Carrie met at camp as kids. When Doug tells his family, his cousin Danny says that it wasn't Doug that Carrie fell on, but it was him. Doug was at “fat” camp that year. In the meantime, Arthur can not pay for the wedding and lets Doug's parents pay. He spends way too much money. Doug can not get the courage to tell Carrie the truth about the story. This story is the only thing that…
When Doug and Carrie's plane makes a rough landing, Doug takes the oxygen mask all for himself. However, their plane was on the ground. Carrie is upset that Doug would not help her if she was in danger. Now Doug is trying his hardest to think of a time when he helped Carrie. Meanwhile, Spence moves in with Doug's cousin Danny.
Carrie's boss requests her to get Doug to be an expert witness in a court case, however Carrie feels unsure on how he will do. Spence and Danny have a house warming party because they just moved in together. Danny meets Holly he wants to ask her out, but Spence says he can't.
It's Christmas and Carrie get another negative pregnancy test. So Doug and her go to a fertility doctor and they find out the reason they are having trouble is because Carrie only has one Ovary working. The doctor also tells them that they have a few fertile days coming so they should get going. The only problem is Doug's parents are coming to visit for the holiday.
Carrie grows jealous when Spence's girlfriend recruits Doug to test her culinary work.
Deacon asks Doug to come along to Atlantic City for a weekend. Carrie wants to go salsa dancing with Doug but concurs and they go to Atlantic City. However, Deacon has send a message that he won't be coming. Doug's boss on the other hand, did come to Atlantic City and Doug and Carrie don't seem to get rid of him.
Deacon and Kelly are still separated and Doug takes a very depressed Deacon to some clubs. Doug really gets in the groove and makes a habit of it, with or without Deacon.
When Doug and Carrie find an old check in a wedding present, they need to invite Doug's annoying relatives to write them a new one. The Heffernans plan on going to New Orleans with the money. However, Doug's relatives say that New Orleans is “their town”, and tell Doug and Carrie not to go without them. Doug and Carrie get to the “Big Easy”, but can they get back without them knowing.
Carrie's work has an annual golf trip, but Doug and Carrie used to always take Deacon and Kelly. Now that Deacon and Kelly have split up, Doug and Carrie have to try and find another couple to invite to the golf trip. But Deacon later meets a woman and now he can go on the trip and bring his new girl friend along too. Carrie meets his girlfriend and has a hard time clicking with her. Holly agrees to watch Deacon's kids, but she has to bring a reluctant Arthur along.
Uncle Stu is willing to pay for Doug's dream: his own sandwich shop! Carrie doesn't think this is a good idea but is afraid to tell Doug.
Doug's parents are visiting for a week while Carrie is on a business trip. At his work, Doug learns all personnel will have to take a test.
Doug discovers Carrie is much more fun if she drinks a cocktail, or two…
Doug and Spence have a high school reunion, but Spence doesn't want to go because he had a relationship that ended really badly. Doug is looking forward to it, until Carrie starts wearing a new hairstyle, which is a tight bun that looks like a librarian's hair. So now Doug is afraid that she will wear her hair like that to his reunion.
Doug and Carrie donate enough money to Kirby's school library to become patrons of it. However, on the plaque they're only listed as friends.
Doug and Deacon bet 100 dollars on a fight that they are not allowed to bet on. They win 5,000 dollars, and have to get rid of it quick or Carrie will know Doug disobeyed her orders not to take the money. Doug and Deacon decide to take off from work for a whole day and spend all the money. The money is spent on fancy dining, expensive cars, and a live living room performance by the ‘money man’ Eddie Money.
When it turns out that Doug's new dentist once had a crush on Carrie, Doug thinks he hurts him on purpose.
The fourth season concludes with Arthur visiting a psychiatrist and recalling via flashback a disturbing encounter with his dad that may explain Arthur's abrasive personality. Carrie and Doug decide to “validate” Arthur in everything he does. This leads to Carrie visiting the psychiatrist, who also has a flashback that makes her doubt her personality. Which leads Doug to also visit the psychiatrist and has his own flashback that makes him learn something about himself.