You sure the area is correct? Where is it?
You didnt describe what you wanted the code to do.
The code of yours applies the spell 57673 on the player when he has completed the quest if he is in the area 4 and if he already does not have 57673 spell on him.
In my mind the last part seems like a contradiction.
Also, it seems that normal spellcasting rules apply to the aura. If you try to aura a spell that for example the player already has on cooldown, it doesnt work and is not aurad.
The spell of yours requires the player to be in a specific area in itself. So likely using it through spell_aura would not work unless you are in the said area.. possibly.