ML degree formula for the MA(1) model

Let $MA(1)$ be the moving-average model with $n>1$ sample points, and let its maximum-likelihood degree be the number of complex critical points of the likelihood function for generic data. ML degree conjecture. The ML degree of $MA(1)$ for $n>1$ sample points is equal to

$$4(n-1).$$

The formula is inferred from ML degree computations for $n=2,3,\dots$; the supplied text gives no proof or resolution, so the general claim remains open.