Many thanks for your swift reply - I guess my question would be better phrased as 'Can I use a non-parrity protected drive in the same unraid system as 3 drive parity protected group?'.
From your answer it looks like the Unassigned Devices plugin will enable what I need but it's not possible with vanilla unraid? I'm slightly wary of relying on a third party plugin (that is recommended to be sourced from a 134 page forum post!).
From here: https://wiki.unraid.net/index.php/UnRAID_Plugins_for_V6
to the two forum posts below: