Preview at WWDC likely in June, followed by September launch.
iPhone 6s, iPhone 6s Plus and 4-Inch iPhone 6c Rumored for 2015 Release
Apple will release three different iPhones in the second half of 2015, the iPhone 6S, iPhone 6S Plus and a 4-inch device currently being referred to as iPhone 6C, according to industry sources.A new 4-inch iPhone was also rumored in December 2014, with a report claiming that the device would be aimed toward female users. However, even if a new 4-inch iPhone isn't introduced, Apple will presumably have the iPhone 5s to offer for free on a two-year contract once the current models slide down.
DigiTimes notes the 4-inch iPhone 6c is likely to be manufactured by Wistron while both the 6s and 6s Plus will be manufactured by Foxconn. In January, a report from Chinese site Feng.com claimed that Apple had not made any orders in its supply chain for a 4-inch iPhone.
Earlier this year, CEO Tim Cook said the new, larger iPhone models had brought the highest Android switcher rate over the past three years and that they brought in more newer customers than previous iPhone models. It's possible that Apple believes offering more sizing choices to customers could allow Apple to reach more customers, as Cook also mentioned different regions preferred different sizes.
Apple is expected to announce new iPhone models in September, with new additions to both the iPhone 6s and 6s Plus rumored to include 2GB of RAM and Apple's new Force Touch technology, which has already made its way into Apple Watch, the new MacBook and new 13-inch MacBook Pros.