The popularity of the iPhone 4S and new, lower prices on older iPhones are helping Apple narrow the smartphone sales gap between iOS and Android in the U.S., according to the latest data from consumer research firm the NPD Group.

According to a factsheet the company released at the Consumer Electronics Show in Las Vegas, the hotly anticipated iPhone 4S noticeably boosted iOS’ market share of smartphone sales to 43 percent in October and November, up from 26 percent in the third quarter. Apple reported it had sold more than 4 million iPhone 4S’ in its first weekend of the device’s availability.

While the jump between the third and fourth quarter seems dramatic, it is important to note that Apple’s market share in the third and fourth quarters was artificially distorted by the 4S, as many of those those planning to buy an iPhone waited for the new device’s release in October.  Overall however, what was good for iOS proved to be bad for Android, with the popularity of the 4S and the new lower prices on Apple’s older iPhones cutting Android’s share of new smartphone sales to 47 percent in October and November, down from 60 percent in the third quarter.

NPD’s figures also indicate new smartphone sales are now almost exclusively a contest between iOS and Android. RIM’s BlackBerry devices continued to lose market share in the fourth quarter, accounting for just six percent of new smartphone sales. At the beginning of the year, BlackBerry devices were almost as popular as iOS devices, holding 21 and 23 percent of sales respectively.  In response to its declining presence the Canadian company has made a variety of changes, and will soon be licensing BlackBerry’s security features to other manufactures. The company’s co-chairmen, Mike Lazaridis and Jim Balsillie are under intense pressure to step down.

Meanwhile Microsoft’s Windows Phone 7 and Windows Mobile offerings continued to remain marginal, accounting for about one percent of total sales each.

Smartphone sales are also continuing to increase at the expense of traditional feature phones, and now account for 67 percent of all new device sales in the U.S. The top three selling smartphone phones in the fourth quarter were the iPhone 4S, the iPhone 4 and the iPhone 3GS according to NPD.