Unique Rectangle Type 4
When two rescue cells on the same side of a Unique Rectangle form a conjugate pair for one of the deadly-pair digits, that digit is locked in — and the other deadly-pair digit can be removed from both rescue cells.
What is Unique Rectangle Type 4?
Type 4 is the mirror image of Type 2. Instead of an extra candidate being locked into the rescue cells, one of the deadly-pair digits itself is locked in via a conjugate pair. Two rescue cells on the same side of the rectangle are the only remaining candidates for one of the deadly-pair digits in their shared house — a conjugate pair. One of them must hold that digit, which means neither can hold the other deadly-pair digit, because that would complete the deadly pattern.
The elimination is the reverse of Type 1: in Type 1 the deadly pair is stripped from the rescue cell; in Type 4 one deadly-pair digit is confirmed in the rescue cells and the other is stripped from them. The conjugate pair is what makes this work — without it, neither deadly-pair digit would be forced, and the rectangle would not yield an elimination.
How to spot it
Find a rectangle with two rescue cells on the same side, then check whether those two cells are the only candidates for one of the deadly-pair digits in their shared row or column. If they form a conjugate pair for that digit, the digit is locked into them — one of them must be it. That leaves no room for the other deadly-pair digit in either rescue cell, so it can be removed from both. This type is rare because it requires a precise conjugate-pair alignment inside the rectangle, but it delivers a clean double elimination when it appears.
The logic
The conjugate pair means one of the two rescue cells must contain the locked digit. Suppose the other deadly-pair digit also appeared in one of the rescue cells. Then that cell could hold the other digit, the remaining two rectangle corners would be forced to fill the deadly pair, and the rectangle would collapse into the deadly pattern — two interchangeable solutions. Because the puzzle has a unique solution, this cannot happen, so neither rescue cell can contain the other deadly-pair digit. The conjugate pair supplies the lock; the uniqueness guarantee supplies the ban.