Apple’s iPhone 4 maintained its position as the best-selling smartphone for both AT&T and Verizon Wireless last month despite the fact that the handset is now more than a year old. Canaccord Genuity analyst Mike Walkley on Tuesday raised his price target on Apple stock to $545 from his prior target of $515, reiterating a Buy rating. Walkley reports that checks again showed strong sell-through for the iPhone 4 in August, and he also noted that the addition of Sprint as a likely carrier partner for the iPhone 5 will bolster sales in 2011 and into 2012. “Our August checks indicated the iPhone 4 remained the top selling smartphone at AT&T and Verizon despite consumer speculation regarding the upcoming iPhone 5 launch,” the analyst writes. “Further, we believe Sprint will receive the new iPhone at launch and heavily promote it given the carrier’s unlimited data plan.” Walkley now believes Apple will sell 83 million iPhones in 2011 and 107 million units next year, and he bumped his 2011 iPad sales estimates from 56 million units to 59 million as well.