The new device -- called the iPhone SE -- will debut at $399 for US customers without a contract subsidy, in a significant cut from the price of its larger iPhones.
Apple vice president Greg Joswiak said many consumers prefer smaller handsets, which accounted for 30 million units sold in 2015.
"They want the most compact iPhone design," he told an Apple event in Cupertino, California.
"Some people asked us, pleaded with us to keep the four-inch iPhone in our lineup."
The iPhone 6S, the lowest-cost handset in the large-screen family, starts at $649.
Apple will be taking orders Thursday for the new phone in the United States, Britain, Australia, Canada, China, France, Germany, Hong Kong, Japan, New Zealand and Singapore, with more markets to come online in April.