Crossover design in the WM-4 is straightforward because the drivers have essentially flat frequency responses, eliminating the need for any large compensation.
The basic topology is fourth-order Linkwitz Riley. This refers, of course, to the acoustic behaviour, not the electrical filtering. Drive units have their own inherent roll-offs which the electrical filter must account for to achieve the desired acoustic filter response.
A fourth-order Linkwitz-Riley alignment allows us to connect all the drive units with the same acoustic polarity, achieve an essentially flat on-axis frequency response, and ensure predictable results off-axis.