Unique Loop Type 4
This deduction assumes the puzzle has exactly one solution and that all earlier candidate removals were sound. A Type 4 Unique Loop starts from a candidate state in which swapping two digits across the construction would preserve every touched row, column and box. A conjugate restriction on one base digit inside the rescue cells’ shared house removes the other base digit from both.
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: an even loop of unsolved cells sharing the same two base digits, with exactly two loop cells in every house it touches. Exchanging those digits around the loop must move no given and must leave each affected house with one occurrence of each base digit. Type 4 then applies its additional rescue condition; an arbitrary even cycle is not enough.
In Type 4 a house shared by two rescue cells has one base digit confined to those cells. That locks the digit into the rescue cells, so the other base digit is removed from both.
How to spot it
Verify an even loop in which each affected house contains exactly two loop cells and the two base digits can be exchanged around the loop. In a house 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.