** Detailed instructions for working a group puzzle of type 'Sudoku-4' **

The 4 elements in each group of this type of puzzle are: **1**, **2**, **3**, and **4**.

Every group in the puzzle must contain *exactly* this set of elements.

In a Sudoku-4 puzzle, each row is a group numbered as shown in the figure above.

In a Sudoku-4 puzzle, each column is a group numbered as shown in the figure above.

In a Sudoku-4 puzzle, each 2x2 square is a group numbered as shown in the figure above.