you should reroll at 5* because it will change when it goes to 5*. if you are playing the previous halloween event just use those 5* units to reroll.
The arrows change..? I thought advancing job only add new arrow? o_o (need to reconfirm this)
Edit: Confirmed by HungryPanda, thank you!
ops that was really ambiguous but ya hungry panda pointed that out.
either wait for 5* to get an extra arrow to see if its the one you want. if not, reroll then. though its 50-50 chance though so its possible you will get what you want