time-series

Statement as per the book official GARP book (chapter 11, seasonal differencing): How is this equation MA(1)? Isnt this an MA(4) with theta=-1? And how is this covariance stationary? The characteristic equation is 1+z^4 with complex root of abs value of unity.

Hello, I would like some further clarification on the log-linear function that appears on page 5 of the Ch.11 materials: GARP's Ch.11 materials writes the formula differently so i'm having some trouble reconciling. Forgive me if this should be something basic or intuitive (I don't remember...