BUG Type 4
This deduction assumes the puzzle has exactly one solution and that all earlier candidate removals were sound. BUG Type 4 combines a verified BUG guardian group with a second digit restricted to the same exception cells in a shared sector. The conclusion is a candidate restriction unless the displayed state separately proves individual placements.
What is BUG Type 4?
Verify the complete BUG state produced by hypothetical removal of the guardian digit. Then verify that another digit is bilocal in the exception cells’ shared sector. These two at-least-one conditions restrict what the cells can contain; the number of exception cells is an example condition, not a shortcut for the proof.
Type 4 combines the guaranteed BUG guardian alternatives with another digit locked into the relevant exception cells in a shared sector. These two restrictions may remove other candidates, but they do not by themselves determine which exception cell receives which of the two digits.
How to spot it
Identify every guardian occurrence and check the hypothetical BUG counts. In a sector shared by the relevant exception cells, find another digit with no positions outside them. Apply only eliminations supported by both restrictions.
The logic
At least one exception cell must use the guardian digit to avoid the BUG, and at least one must use the locked digit because that digit has no other position in the sector. This can exclude other candidates as the exact cell sets permit, but it does not automatically determine which cell receives which digit.