The Weeds Season 4 On November 5, 2007, Showtime ordered 13 new episodes for a fourth season of Weeds.[12] The fourth season started on Monday, June 16, 2008 and concluded on Monday, September 15, 2008.[13] The season opener "Mother Thinks the Birds are After Her" was the last episode with "Little Boxes" as the theme song. The opening credits of subsequent episodes, after a recap of previous episodes, begin with a video title card unique to each episode. For the second episode of the season, for example, a highway sign reads "Weeds", "Created By", and "Jenji Kohan"—the embedded text used for all title cards since the show's inception. Each title card also has a prop or part of the setting morph into a marijuana leaf. In the highway sign example, a downward arrow designating a lane becomes a slightly larger pot leaf. Silas is now 17 and Shane is 13.

The Weeds Season 4 having lost both her Agrestic grow house and her residential house in fires, Nancy relocates her family to the fictional California town of Ren Mar, near the Tijuana-San Diego border. They move in with Andy and Judah's father in Ren Mar. Guillermo hires Nancy to bring by car what she believes to be illegal drugs from Mexico, but, as she finds out after delivery, they are actually legal asthma inhalers used to test Nancy's smuggling ability. Celia is in jail due to being the official lessee of Nancy's grow house where she bargains to spy on Nancy in exchange for her release. Guillermo's men catch Celia spying but Nancy convinces them to spare Celia's life. Andy enters a coyote partnership with Doug, who has recently moved to Ren-Mar to evade questions about Agrestic's finances. Isabelle is unenthusiastic about moving from Ren Mar to her father in Detroit and pesters her mother Celia until she lets her daughter stay with her. Silas sets up a grow room in the rear of a gourmet cheese shop owned by a neighbor (Julie Bowen) who is an attractive thirtiesh woman. Despite knowing that Silas is under 18, she becomes intimate with him. She reveals to Silas that her interests in him are just financial and physical, causing a heartbroken Silas to spurn her advances and to end their business relationship. Meanwhile, at school Shane attacks without provocation the most popular boy to acquire a fearsome reputation. He also attracts the admiring attention of Simone and Harmony, with whom he loses his virginity in a threesome.

Guillermo's boss, who has so far remained unknown to Nancy, has her open a maternity store which she believes is solely for money laundering. But she finds in the back of the store an underground tunnel entrance, and is told it is for transporting marijuana from Tijuana. Nancy later learns that the tunnel helps transport other controlled substances as well as guns and women. Shocked, Nancy informs DEA Captain Roy Till of the tunnel, though she has begun a sexual relationship with Guillermo's crime boss, the now revealed Esteban Reyes (Demián Bichir) who is also Mayor of Tijuana. The resulting DEA raid and shootout at the maternity shop ends with most of the Mexican drug runners arrested.

In Weeds Season 4, while working at Nancy's store, the availability of drugs from the tunnel leads to Celia abusing them. Isabelle and Dean stage an intervention which spurs Celia to enter rehab and to make amends to her family. Dean rejects her attempted amends, insisting that she instead locate and make amends to their oldest daughter, Quinn, who departed during the show's inaugural episode for a Mexican boarding school named Casa Reforma. Quinn, newly graduated, has her mother drugged upon her arrival and held captive with the aid of Quinn's boyfriend, a self-styled Mexican revolutionary, in order to extract a ransom of $200,000.

Esteban discovers that it was Nancy who had alerted the DEA to his smuggling tunnel, captures and intends to have her killed. But after Nancy discloses that she is with his child, a likely boy (he has only daughters), he spares her. At home, Shane steals Silas' weed, and Simone and Harmony help him to sell it at school. Doug had fallen for an illegal alien he names "Mermex" as she unsuccessfully tried to enter the United States by beach. After much searching, Doug locates her and uses his coyote enterprise to get her into California. Instead of gratefully rewarding Doug's efforts, Mermex is repelled by his unapologetic nature and genital warts. She has instead fallen in love and slept with Andy, now a folk hero known as "El Andy." A scorned Doug turns Mermex into immigration. Realizing that poverty effectively denies him from having to pay any judgments, Doug dares in writing for his estranged wife daring her to take him to court and then appears to prepare to hang himself, but actually masturbates using auto-erotic asphyxiation. Andy wonders if he is in love with Nancy and questions why he has stayed with the family for so long.