In this paper, we consider the non-spectrality of the planar self-affine measures μM,D generated by an expanding integer matrix M∈M2(ℤ) and a four-element digit set
D={(00),(α1α2),(β1β2),(−α1−β1−α2−β2)}with α1β2−α2β1∉2ℤ.
We show that L2(μM,D) contains an infinite orthogonal set of exponential functions if and only if det(M)∈2ℤ. Moreover, if det(M)∈2ℤ+1, then there exist at most 4 mutually orthogonal exponential functions in L2(μM,D), and the number 4 is the best.