I have a level 13 Druid with access to the Cloud (Air Domain). My level 7 domain spell is Elemental Body IV (Air). Can I cast that while I am in a Wild Shape? Or does polymorph rules prevent that?
You can only have one of those effects active at a time.
Wild Shape "functions like "the beast shape I spell, except as noted here."
You can only be affected by one polymorph spell at a time. If a new polymorph spell is cast on you (or you activate a polymorph effect, such as wild shape), you can decide whether or not to allow it to affect you, taking the place of the old spell. In addition, other spells that change your size have no effect on you while you are under the effects of a polymorph spell.
Because of this, whichever effect, Wild Shape or spell, is activated second takes precedence (although you can decide to ignore it). Ask your GM about how they interpret "taking the place of the old spell".