Line 4, also known as Trinitat Nova – La Pau, usually called "línia groga", 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.

The Barcelona Metro is a network of rapid transit electrified railway lines that run mostly underground in central Barcelona and into the city's suburbs. It is part of the larger public transport system of Barcelona, the capital of Catalonia, Spain, with unified fares under the Autoritat del Transport Metropolità (ATM) scheme. As of 2014, the network is operated by two separate companies: Transports Metropolitans de Barcelona (TMB) and Ferrocarrils de la Generalitat de Catalunya (FGC). It is made up of 12 lines, combining the lines owned by the two companies. Two lines, L9 and L10, are being built at present, with both lines having different sections of each opened between 2009 and 2018. They are due to be fully completed in 2026. Three lines on the network have opened as automatic train operation/driverless vehicle systems since 2009: Line 11 being converted to driverless first, and then Lines 9 and 10, opening up driverless.