Reduction and Yau

Centres and edges reduce the 4x4 to a 3x3; two parity cases a 3x3 cannot have.
Read the overview: 4x4: reduction, Yau and parity

1. Reduction

Centres and edge pairing

Concept
Make it a 3x3.

2. Parity

OLL parity

3 cases
One dedge flipped: 3 cases.

PLL parity

22 cases
Two pieces swapped: 22 cases.