Line 4, also known as Trinitat Nova – La Pau, usually called "línia groga" (yellow line), is a line in the Barcelona Metro network operated by TMB, and part of the ATM fare-integrated transport network. It serves the northern districts of the city, and it is being extended to the new major metro and rail stations Estació de la Sagrera and Sagrera-Meridiana.

Opened to the public in 1973, it serves the northern half of Barcelona using part of the infrastructure of Barcelona's first metro line the Gran Metro de Barcelona, covering a wide C-shaped area stretching from La Pau (in la Verneda) to Trinitat Nova, where it is linked with the recent L11. The 16.7 kilometres (10.4 mi) that make up the whole line are underground.

Although in 1966 it was planned for line 4 to be a loop line connecting Trinitat Nova and La Pau stations with three intermediate stations, in 1974 the plan was subsequently changed such that it became a C-shaped line of today, with provisions for two extensions northeast of Trinitat Nova (to link it with line 3 at the border of Barcelona and Santa Coloma de Gramenet) and La Pau stations (intended to penetrate inside Santa Coloma de Gramenet) that have never been realised.

• 1973 – Joanic-Jaume I section opened (part of it had been part of L3 previously)
• 1974 – Joanic-Guinardó section opened
• 1976 – Jaume I-Barceloneta section opened
• 1977 – Barceloneta-Selva de Mar section opened
• 1982 – Guinardó-Via Júlia and Selva de Mar-La Pau sections opened.
• 1985 – La Pau-Pep Ventura section opened
• 1999 – Via Júlia-Trinitat Nova section opened.
• 2002 – La Pau-Pep Ventura section closed (and moved to L2)
• 2003 – El Maresme-Fòrum station opened.

District	Station	Opened	Connections
Nou Barris	Trinitat Nova	1999	Barcelona Metro: L3, L11
	Via Júlia	1982
	Llucmajor	1982
Horta-Guinardó	Maragall	1982	Barcelona Metro: L5
	Guinardó | Hospital de Sant Pau	1974
	Alfons X	1974
Gràcia	Joanic	1973
Eixample	Verdaguer	1970	Barcelona Metro: L5
	Girona	1973
	Passeig de Gràcia	1973	Renfe Media Distancia: R11, R13, R14, R15, R16 Rodalies de Catalunya: R2, R2 Nord, R2 Sud Barcelona Metro: L2, L3
	Urquinaona	1926	Barcelona Metro: L1
Ciutat Vella	Jaume I	1926
	Barceloneta	1976	Renfe Operadora: Euromed, Talgo (at Barcelona França) Renfe Media Distancia: R13, R14, R15, R16 (at Barcelona França) Rodalies de Catalunya: R2 Sud (at Barcelona França)
	Ciutadella | Vila Olímpica	1977	Trambesòs: T4
Sant Martí	Bogatell	1977
	Llacuna	1977
	Poblenou	1977
	Selva de Mar	1977	Trambesòs: T4
	El Maresme | Fòrum	2003	Trambesòs: T4
	Besòs Mar	1982
	Besòs	1982	Trambesòs: T5
	La Pau	1982	Barcelona Metro: L2
	Santander
Sant Andreu	Sagrera-TAV		Renfe: AVE (planned) Renfe Media Distancia (planned) Rodalies de Catalunya (planned) Barcelona Metro: L9, L10 (planned)
	La Sagrera-Meridiana		Renfe Media Distancia: R12 FGC: R3, R4 Barcelona Metro: L1 L5, L9, L10

• "(home)". Trenscat.

• Media related to Barcelona Metro line 4 at Wikimedia Commons

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