That is like saying 'select 4 lists that you (approval) think appropiate', i.e. in your game you could think that any essence users could learn 'elemental attacks' so any essence pure spell user could choose any magician list as base.
Another example, for channeling our group could aprove that any priest (from channeling companion) list associated to your deity is available as extra base list.
As that is a subjective decision (which lists are appropiated?) a generalized way to this is saying:
A pure spell user may select any FOUR lists in his/her realm to be classified as base lists along with those normally associated with his/her profession.
In my game we can choose open/closed/evil lists, the usual.