The four-disk configuration is identical to the three-disk configuration, with the addition of a fourth disk, which is a hot spare for the mirror.
This configuration minimizes the time window between the failure of one of the mirrored disks and its replacement, as this now happens automatically. Although the time window is minimized, it is not eliminated because the failure of the remaining disk, before the hot spare is completely synchronized, is still fatal.
There is an argument for triple mirroring instead of configuring a hot spare disk. The third mirror can be seen as a hot spare that is already synchronized, and the time window, during which the system is vulnerable, is effectively reduced to zero. On the other hand, there may be a performance impact when writing intensively to the mirror. We leave the choice to you.