I will show you how to solve 4×4 Sudoku problems. 4x4 sudoku problems are not very difficult, so it might be fun to try and figure it out on your own at first.
An introduction to the 4x4 Sudoku app is here.

Solution 1

Look for three numbers in a horizontal row. You can see the number of the remaining 1 cell that does not contain a number.

Look for three numbers in a vertical row. You can see the number of the remaining 1 cell that does not contain a number.

Find 3 numbers in a 2x2 block. You can see the number of the remaining 1 cell that does not contain a number.

Solution 2

Focus on the lower left block in the figure below. Since there is a 4 in the green ○ cell, you cannot put a 4 in the green × cell. In the lower left block, the only cell that can be placed is the cell with red ○, so you can see that 4 will fit in the cell with red ○ cell. Similarly, other blocks and other orientations can be solved in this way.

Solution 3

Notice the top row in the diagram below. Since there is a 3 in the green ○ cell, you cannot put a 3 in the green × cell. In the top row, there is only red ○ cell that can be placed is the cell with red ○, so you can see that 3 will fit in the red ○ cell. Similarly, other rows and columns can be solved in this way.

Summary

The 4x4 number place problems can be solved with these three methods, regardless of the orientations and positions.
Can you solve all 108 problems?