Divide the diagram along the grid lines into regions of exactly 4 cells. A number in a cell indicates how many of the four edges of the cell are used by region borders (the border of the diagram is also count).

Example puzzle

Grid control: To draw a cut click at the cell edge or move mouse along the line holding the button. You can also connect the centers of cells which belong to the same part with auxiliary line. In this case cut lines will be added automatically. More details about grid control are at the help page.