U.S.-based investment information firm Valuewalk informs that a Samsung Galaxy S9 Mini version is most likely to be launched in early 2018, along with the S9 and S9+ flagship models. The Mini model would get a 4-inch display with curved edges and all the other powerful specs that come with the larger versions.
https://www.notebookcheck.net/An-early-2018-Samsung-Galaxy-S9-Mini-is-highly-likely-according-to-new-rumors.263838.0.html