Sudoku X4 is made by the Sporadic Games group of Magnolia Heights R&D.
Website: sudokux4.com
Found a problem? The easiest way to tell us is from inside the app. Turn on Bug reporting in Settings, then tap the red ! button on the screen where it happened. The report carries a picture of that screen and the puzzle as text. Or write to bugs@sudokux4.com.
The title bar of the album's front page shows which version you have, for example “Sudoku X4 (v2.13.1 build 6030)”. The exact build, with its date, is in small print at the very bottom of Settings.