Unique Loop Type 4
This deduction assumes the puzzle has exactly one solution and that all earlier candidate removals were sound. This Type 4 Unique Loop deduction starts from a candidate arrangement whose two-value swap would preserve every affected row, column, and box. Extra candidates or a link restriction prevent that deadly swap and justify only the stated elimination.
What is Unique Loop Type 4?
This deduction assumes the puzzle has exactly one solution and that all earlier candidate removals were sound. First verify the complete two-value structure and every sector it touches. The swap must move no given and must leave each affected sector with one occurrence of each base digit. Type 4 then uses its additional candidate condition; an arbitrary rectangle or even cycle is not enough.
Type 4 uses the additional condition described below to prevent the verified two-value exchange. The conclusion may be inside the rescue cells or in their common peers; apply only the target rule for this type.
How to spot it
Verify an even loop in which each affected sector contains exactly two loop cells and the two base digits can be exchanged around the loop. In a sector shared by the rescue cells, one base digit is restricted to those cells. That restriction prevents the other base digit from occupying either rescue cell; do not claim the individual allocation of the locked digit unless another deduction proves it.
The logic
The locked base digit must occur in one rescue cell. If the other base digit occupied either rescue cell, the remaining base digits could complete the forbidden exchange. Therefore remove the other base digit from both rescue cells; the locked digit’s exact cell may remain unresolved.