When four cells in a region together hold only four digits, those digits belong to those cells.
Where to lookA pointer first: look at row 9. Where can 4 still go?Locked candidatesIn row 9, 4 can only go in column 5 or column 6, and those two cells lie in the bottom box. So no other cell in the bottom box can be 4.Locked candidatesIn column 8, 8 can only go in row 1 or row 2, and those two cells lie in the top-right box. So no other cell in the top-right box can be 8.Hidden pairIn the top box, 1 and 9 can only go in row 1, column 5 and row 3, column 4, which fills those cells. So they can't be 2, 3, or 6.Naked quadIn the top-left box, row 1, column 1, row 2, column 1, row 3, column 1, and row 3, column 3 can only hold 2, 5, 6, and 9, so those digits must go in those cells. No other cell in the top-left box can be 2 or 6.
Look in a row, column or box for four cells whose candidates, taken together, are only four digits. Each cell need not have all four. Remove those digits from the region's other cells.
This example, step by step
Where to lookA pointer first: look at row 9. Where can 4 still go?
Locked candidatesIn row 9, 4 can only go in column 5 or column 6, and those two cells lie in the bottom box. So no other cell in the bottom box can be 4.
Locked candidatesIn column 8, 8 can only go in row 1 or row 2, and those two cells lie in the top-right box. So no other cell in the top-right box can be 8.
Hidden pairIn the top box, 1 and 9 can only go in row 1, column 5 and row 3, column 4, which fills those cells. So they can't be 2, 3, or 6.
Naked quadIn the top-left box, row 1, column 1, row 2, column 1, row 3, column 1, and row 3, column 3 can only hold 2, 5, 6, and 9, so those digits must go in those cells. No other cell in the top-left box can be 2 or 6.
In hints
The hint marks the four cells and their digits, and strikes those digits from the region's other cells in orange.