Uber first hit the road in London four years ago, and today said it has more than 2 million regular users in the British capital.
To mark the birthday, uberPOOL rides -- which are shared with other passengers going in roughly the same direction -- within zones 1 and 2 will cost a flat rate of £4 for Wednesday only. Enter "LONDON4" into the app to get the discount. The offer is available until midnight.
Although Uber is celebrating how popular it has become with London users, the ride-sharing app has been a source of huge controversy in the city that's home of the famous black cab. As in many cities around the world, the local taxi industry has accused Uber of riding roughshod over established regulations and has blamed local authorities for allowing the service to operate.
Failed mayoral candidate George Galloway threatened to "run Uber out" at the most recent of several demonstrations by cab drivers in London, but authorities have continued to back the controversial app. In an olive branch to the taxi industry, Uber offers black cab drivers the chance to drive for the app without service fees.
Launched in San Francisco in 2011, Uber has established itself in London and other cities around the world with aggressive pricing, although surge pricing means the cost of a ride can be dramatically inflated at times of high demand. By contrast, London's black cabs charge a flat rate set by regulators. Other apps that allow you to hail a black cab or private hire car from your phone in the UK include Gett, Hailo and Karhoo.