You cannot destroy aggr0
You cannot remove a disk from an aggregate.
Assuming that your FAS isn't yet hosting any data :
One solution would be : (summary)
* disable takeover ( cf disable)
* disable automatic disk assignment ( options disk.auto_assign off) on both controllers.
* reboot controller2 and perform a re-installation from the special boot menu , this will use only 3 disks and let the other one unowned ,thus assignable to controller1.
Another solution would be : (summary)
As you have 2 spares , you could assign the controller1 spare to controller2 , build a new raid4 aggregate on ctl2 with the 2 spare disks , move vol0 on the new aggregate , promote this new aggregate as root aggr , reboot , then destroy the old aggr0 and reassign all but two disks to controller 1.
You will have to change some defaults options like minimum amount of spares, ...
Warning : these are advanced tasks , if you are not (yet) confident with NetApp , I recommend you to call your IT integrator or even NetApp Professional Services.