The R4 is a line of Rodalies de Catalunya's Barcelona commuter rail service, operated by Renfe Operadora. It runs northwards from the northern limits of the province of Tarragona to Barcelona, passing through the inland Alt Penedès region. The line then continues towards central Catalonia, describing a U-shaped route through the Barcelona area. According to 2008 data, the line's average weekday ridership is 105,935, the highest on any line of the Barcelona commuter rail service after the R2.

Rodalies de Catalunya is the main commuter and regional rail system in the Spanish autonomous community of Catalonia. It is administered by the Government of Catalonia and operated by the national rail operator Renfe Operadora. The system consists of 17 service lines chiefly centred in the Barcelona area, serving a total of 203 stations throughout Catalonia, with an average number of 1,000 trains running on it every day. In 2016, it had an annual ridership of 117 million.