Apple is expected to sell as many as 4 million units of its latest smartphone, the iPhone 4S, this weekend as big swarms of Apple fans have been hitting the Cupertino, Calif.- based gadget maker's stores around the world to grab one of the last products designed under Steve Jobs, who died last week.

Most estimates for weekend sales of the iPhone 4S range between two to three million, but Yankee Group analyst Carl Howe believes that Apple could sell up to 4 million units.

The estimated sales figure for the iPhone 4S in the first weekend is more than double of the iPhone 4's first weekend sales of around 1.8 million units.

The latest version of iPhone went on sale on Friday in a number of countries across the world, including the United States, Canada, Britain, France, Japan and Australia.

At its launch, the iPhone 4S was described as an iPhone 4 with an A5 chip. But, it also has many other striking features such as an 8-megapixel rear-facing camera, and new software called Siri Assistant that lets users to control their handsets using their voice.

Shares in Apple gained 3.3 per cent to $422 on Friday, representing a gain of 14 per cent this week, and making the company the world's most valuable business with a market capitalization of more than $391 billion.