401: Unwritten Rules
When an armed robbery ends with the death of a police officer, Frank makes it a mission for the NYPD to bring the suspect to justice. However, tensions run high in the Reagan household when Erin drops the charges and releases the suspect due to lack of evidence, on the fourth season premiere.
409: Bad Blood
Danny and Baez investigate a homicide in which Baez's estranged brother, a former drug addict, is the lead suspect. Meanwhile, Frank learns one of his most highly decorated officers, a German Shepherd in the K9 unit, is being accused of police brutality, and Henry investigates the widow of a former NYPD pal who might have had a hand in her husband's death.
411: Ties That Bind
Danny's loyalty is tested when he is assigned to the case of a childhood friend, Mickey, who is under investigation for having ties to an infamous mob family. Meanwhile, Frank gets word that crime is on the rise due to a recent judicial ruling about the legality of the "stop and frisk policy.
415: Open Secrets
When Danny and Baez investigate the kidnapping of a young girl who disappeared after school, it brings back difficult memories for Danny, who worked on a similar unsolved case many years ago. Meanwhile, Frank butts heads with Inspector General Kelly Peterson when he refuses to divulge information about a program that allows Frank to oversee intelligence officers abroad.
416: Insult To Injury
When Danny takes a call from a woman who is threatening to kill herself along with the man who killed her parents in a drunk driving accident, he rushes to find her. Meanwhile, Frank accompanies his grandson, Sean, on a class field trip and struggles to get the lead chaperone to loosen the reigns so the kids can have fun.
419: Secret Arrangements
Danny and Baez investigate the murder of a respected college professor who was involved with a questionable scholarship program. Meanwhile, Danny discovers that his father didn't tell him that the District Attorney is investigating Danny for ethical and procedural violations.
420: Custody Battle
When Erin is assigned to investigate a case in which an on-duty police officer is accused of killing a suspect already in custody, Frank urges her to stick to the facts to avoid a publicity scandal for the NYPD. Meanwhile, Jamie urges Eddie to forgive her father for his past mistakes.
421: Above And Beyond
When a detective from Danny's precinct is killed while working undercover, Frank puts on a full-court press to find his killer, only to discover that the deceased had a big secret. Meanwhile, Jamie and Eddie help Nicky track down a former classmate who is living on the streets.
When Danny is placed on modified duty after investigating a case that was off-limits, he realizes he stumbled upon something that someone is trying to keep quiet. When he passes the tip on to his father, Frank is alarmed when he learns the enormity of this case and the internal involvement, on the fourth season finale.