The error degrees of freedom is $k^2 - 4(k-1) - 1 = 3$

The error degrees of freedom is $k^2 - 4(k-1) - 1 = 6$

The design requires $k^2$ experimental units and provides 3 orthogonal factors

The model assumes all four factors $\alpha, \beta, \gamma, \delta$ are interacting with each other