This fourth puzzle is not quite as straightforward as the first three, and is rated medium. Some more thinking might be required for this one. Don't forget you can use the "Undo" button to go back if you find you've gone wrong.
As before, use your mouse to left-click to select numbers you're sure of, and right-click for numbers you think are possibles. Use the "Float Window" button to popup a new window which you can make bigger or smaller as you want.
According to Sudoku Sensei, this puzzle is coded as
N2x01-057 which means a single level 2 rule is needed to solve it, and it starts with 57
empty squares. It's this level 2 rule which bumps the complexity up to medium.