Type 4 Unique RectangleA Type 4 Unique Rectangle occurs when one of the digits belonging to the non-unique rectangle forms a conjugate pair in cells that have additional candidates. Thanks to this, we can remove the second digit of the non-unique rectangle from these two cells.
In the example below, a non-unique rectangle can appear in the cells R3C4, R3C6, R5C4 and R5C6 with the digits 5 and 8. The digit 5 in the third row (and also in the second square) can only appear in the cells R3C4 and R3C6. So to prevent the non-unique rectangle, you can eliminate the digit 8 from these cells. After this elimination, the digit 8 in the second square can only be entered in the R1C5 cell.