iPhone SE 2
This device would likely come with a similar 4-inch screen size, though Apple could push it to a slightly larger size if they opted for an edge to edge display.
If Apple hosts a spring event the iPhone SE 2 would likely be a major focal point of the event. Focus Taiwan reported in August 2017 that Apple supply Wistron was prepping for the launch of a new iPhone SE in Q1, which times up with a March release. The Taiwan Economic Daily News reported in November that Apple will release a new iPhone SE in the first half of 2018. In January 2018 another rumor adds to the mounting evidence that an iPhone SE 2 release is part of Apple's plans.
If you are looking for a small iPhone in 2018, the iPhone SE 2 is what you're interested in. Here's why you should wait for the iPhone SE 2, and why you shouldn't.