Having recently crossed the 3 million mark on August 29, location-based startup Foursquare has processed its 4 millionth user registration today, according to a report by the company.

Despite the previous service problems faced by Foursquare earlier this year, the company is now adding close to 20,000 users per day, up significantly from the estimated 15,500 per day rate when it crossed the 3 million mark, Mashable.com reported. "Foursquare's impressive growth can be attributed to a number of things, most notably the release of version 2.0 for iPhone and Android (Android), a Symbian release and celebrity attention from the likes of Conan O'Brien and hip-hop artist Big Boi," Mashable's Jennifer Van Grove explained.

Among other factors that contribute to rapid growth of the startup has been increasing awareness among users in cities around the world and college campuses, along with mentions in mainstream media such as MTV's VMA Awards, Late Night with Jimmy Fallon and People magazine, the company noted. Having recently introduced Facebook Places in mid-August, the company has been seeing a very positive growth rate.

The company has lately been battling competition from Gowalla with around 450,000 users in the beginning of September and figures from adoption of Facebook Places is yet to be revealed, according to Mashable.com.