The alignments for line 4 under the 1966 plan (in black) and 1974 plan (in grey) as well as the extension to Sagrera-Meridiana under construction (in dashed line)
— 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 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.