2

x x  x x
x x  x x

2 x  x 1
x x  4 x

(Multiple solutions.)

https://www.reddit.com/r/sudoku/comments/h9ovdg/worlds_hardest_4x4_micro_sudoku/
Fluttering3600
2020
World's Hardest 4x4 Micro Sudoku
Ant Twin MicroSudoku
By Hippogonal#4902