The STR4 continued the pattern established by its predecessors, using a Red Bull Racing chassis — in this case the RB5, itself a leading car in the 2009 field — mated to a Ferrari V8 engine of the previous year's specification. The use of a year-old engine specification reflected Toro Rosso's ongoing approach of running customer power units at reduced cost. Revealed at a Barcelona testing session rather than a formal launch event, the car was presented with relatively little fanfare.
The STR4 marked the end of an era for the Faenza-based outfit. Regulations had already been changed to prohibit customer chassis arrangements from 2010, meaning this would be the last time Toro Rosso could draw on Red Bull Racing's design resources for its race car.
The STR4 was driven by Sébastien Bourdais and Sébastien Buemi for the first half of the season, before Jaime Alguersuari replaced Bourdais from the Hungarian Grand Prix onwards.
Sébastien Buemi scored a seventh-place finish on his Formula One debut at the Australian Grand Prix, an encouraging introduction to the sport. He added an eighth place in China, and finished seventh and eighth respectively in Brazil and Abu Dhabi, contributing solid points across the year.
Sébastien Bourdais scored an eighth place in Australia alongside his debutant teammate, but his season deteriorated as it progressed. His 2008 results had already been unflattering in comparison to teammate Sebastian Vettel — Bourdais had scored just four points in 2008 to Vettel's 35, including Vettel's race win at Monza. In 2009, Bourdais managed only two points after the German Grand Prix, and was dismissed from the team at the Hungarian round to be replaced by Alguersuari.
Jaime Alguersuari stepped in at the Hungarian Grand Prix, becoming the youngest driver in Formula One history at the time of his debut. His arrival provided little immediate scoring improvement, however, as he managed to finish only three of his eight races that year, with a best result of 14th at the Brazilian Grand Prix.
The STR4's base colour was dark blue, reflecting the primary branding of title sponsor Red Bull, whose energy drink logo and lettering added red accents to the car's flanks.
At the Chinese Grand Prix, Toro Rosso added a decal to the cars reading Vicini All'Abruzzo — a tribute to victims of the 2009 L'Aquila earthquake that had struck the Abruzzo region of Italy, where the team's Faenza base is located.
The STR4 closed the chapter on Toro Rosso's customer chassis period, a four-year arrangement that had been central to the team's identity and the subject of ongoing controversy since the STR2. From 2010, the team would design and build its own cars entirely, a shift that fundamentally changed the nature of the organisation. The STR4 season also accelerated the departure of Bourdais and confirmed Buemi and Alguersuari as the pairing who would take the team into its new independent era.