By Katie Marsal

Published: 10:00 AM EST

After spending 25 hours observing sales of iPhones at Apple's national retail chain, investment bank Piper Jaffray now estimates the company is on track to announce a more than fourfold increase in sales of the handset during its fiscal fourth quarter.

"Our 25 hours of counting iPhone sales in Apple retail stores throughout the country lead us to believe Apple is on pace to beat our previous estimate of 4.1 million iPhones for the September quarter," analyst Gene Munster advised clients in a report Wednesday.

He now estimates that the electronics maker will sell at least 4.47 million iPhones in the quarter, compared to 1.12 million during the same three-month period last year. More specifically, he estimates that Apple sells an average of 95 iPhone 3Gs per day at each of its 188 U.S. retail stores, up from 21 phones per day during a similar check in August of 2007.

"We believe Apple will sell 1.78 million units at its U.S. retail stores, and 0.90 million phones at U.S. AT&T locations," Munster wrote. "Our estimate assumes each of the 2,200 AT&T stores will sell an average of 5 phones per day from the July 11th launch through the end of the September quarter."

The analyst had noted in a previous report that he believes the U.S. sales will account for only 53 percent of iPhones sold worldwide, leaving international sales of the handset to account for the remaining 1.79 million units that make up his fourth quarter estimate.

While demand for the iPhone 3G remains undoubtedly high, Munster's model factors in the likelihood that demand will decrease slightly in the weeks ahead. His firm's 25 hours of checks spanned two weeks, and he noted sales volume decreased about 10 percent during the second week.

Taking this into account, the analyst said his fourth quarter estimate assumes Apple retail stores will average 31 iPhone sales per day for the remaining 51 days in the quarter.

"We believe that our revision may be conservative and that Apple will meet our estimates," he wrote. "The August 22nd launch in additional countries should provide another catalyst to sales."

Munster's model assumed an average of 12 hours of sales per day based on 8 percent of the stores being high-traffic "flagship" locations and remaining 92 percent regular-sized outlets.