The First 2 puzzles are regular sudoku, (despite looks, not SudokuX). In the third puzzle all neighbouring numbers with a difference of 4 are marked with a red circle on the side between them. In the final puzzle one has to use the numbers found in the above puzzles and add clues until the puzzle is solvable.

Here is the 4th puzzle with the clues A-L (anyone free to independently confirm the clues A-L)

- Code: Select all
`A7 . . | B6 . C3 | . . .`

. . . | . . . | . D8 .

. . . | . . . | . . .

----------+---------+--------

. . . | 3E . . | F5 . .

. . G8 | . . . | . . .

. H4 . | . I8 . | . . .

----------+---------+--------

J9 . . | K5 . . | L3 . .

. . * | . . . | . . .

. . . | . * . | . . .

two cells marked with (*) must also be clues.

Is there a logical way to find the minimum number of clues for which this puzzle can be solved, or is it abit of 'Trial and Error' involved