BUG Type 4
BUG Type 4 combines two escape cells sharing one extra candidate with a locked digit in their shared house: one escape cell must hold the extra candidate, so the other must hold the locked digit, and the remaining candidates in both cells can be removed.
What is BUG Type 4?
A valid Sudoku has exactly one solution. Near the end of a solve, the grid can reach a state where almost every empty cell has exactly two candidates. If every remaining cell were bivalue and every candidate appeared exactly twice in each row, column, and block where it occurs, the grid would have two interchangeable solutions — a deadly pattern called a Bi-value Universal Grave, or BUG. Any cell that breaks this symmetry by carrying an extra candidate is called an escape cell.
Type 4 is the case where two escape cells share the same extra candidate, and a second digit is locked into the same two cells in their shared row, column, or block — that is, those two cells are the only remaining positions for that second digit in the house. Because removing the shared extra candidate from both cells would restore the deadly BUG, at least one of the two escape cells must hold it. Since the locked digit has no other position in the house, the other escape cell must hold it. Therefore neither cell can contain any of the remaining candidates, and they are removed from both.
How to spot it
BUG Type 4 appears late in a solve, when the grid is nearly complete and most empty cells are bivalue. Look for two cells that each have three candidates while every other unsolved cell has exactly two, and check whether both carry the same extra candidate. Then look at their shared row, column, or block: if a second digit appears only in those two cells within that house, it is locked into them. The combination of the shared extra candidate and the locked digit forces one cell to take the extra candidate and the other to take the locked digit, so all other candidates in both cells can be removed. If the two escape cells share the same extra candidate but no digit is locked into them, you are looking at Type 2 instead; if they carry different extra candidates, it is Type 3.
The logic
If the shared extra candidate were removed from both escape cells, every other empty cell would be bivalue and every candidate would appear exactly twice in each house — the BUG state, which has two interchangeable solutions. A puzzle with a unique solution cannot contain a BUG, so at least one of the two escape cells must hold the shared extra candidate. Because the locked digit has no other position in the shared house, the other escape cell must hold it. With one cell committed to the extra candidate and the other committed to the locked digit, none of the remaining candidates in either cell can be correct, so they are all removed. The deduction combines the uniqueness guarantee with a locked-digit constraint, producing two simultaneous placements and a clean sweep of the leftover candidates.