Pair hidden quad
Four pairs of one symbol confined to four cells; other pairs of that symbol are blocked at those cells
How to use it
Four pairs of one symbol whose homes all lie inside the same four cells. Whatever the matching, those four cells carry exactly the four pairs; every other pair is blocked at them and every candidate that cannot join one of the four pairs is struck there.
Quads are rare and worth the patience: by the time four pairs share four cells, the block usually collapses something. The scan is the same homes table used for pairs and triples, one row longer — and remember to run it on digits too.
Why it is sound
Four unplaced pairs of one symbol whose homes all lie inside the same four cells fill those four cells in every solution — four pairs, four admissible cells, at most one cell per row and column. Blocking the remaining pairs at those cells removes only configurations absent from every solution.
A worked example
Letter D's pairs D2, D3, D5 and D6 have all their remaining homes inside four cells: r2c3, r4c4, r5c5 and r6c7 — four cells that between them must absorb the four pairs, one pair each. The leftover D-pairs die there: D4 leaves r2c3 and r6c7, and D7 leaves r4c4 and r5c5.