Best 4 person all caster/priest group

[Duik]

Depends.
Meandering permanent group of 4 friends would start knowing that 2 chant 1 clr 1 doesnt matter but not wiz or dru is min/max. Buying expensive spells between them.

Lvl60's doing named camps 2 chant 1 clr 1 doesnt matter but not wiz (unless planes) or dru.

Choose mage if dependable burst dps is a priority.
Choose sham if you want a grouped soloer fencing off part of the sandbox playing with his 4 - 5 "friends" for mage like dps.