Quad Forcing Chains
Quad Forcing Chains are seperated out from the other Forcing Chains and put at the end of the strategies used by the solver. This is done for several reasons: They are rare but take a relatively long time to search for and they are not at all easy to spot as a human so I weight them as beyond the other exotic strategies and therefore part of the last resort.
There is no additional strategy required to spot them except that the starting position will be looking for either four candidates in a cell to make a Quad Cell Forcing Chain
or four remaining candiates of number X left in a unit to make a Quad Unit Forcing Chain
. Examples of each type can be found on those pages.