There is an extra point of damage per arrow in addition to what is stated in the spell's description, except when cast with maximize spell. (That is, damage is actually 4d6+1 per arrow, but each arrow's damage is only 24 when maximized.)
Saving throws and spell resistance checks are made for each arrow of this spell.
Since spell resistance is applied to each arrow individually, this spell is more effective than most when it comes to using up an opponent's spell mantles, as each arrow will reduce that protection by three spell levels. At higher caster levels (16+), this spell can eat more mantle levels with a single casting than any other spell. (This behavior is changed by the set of spell changes that has been linked to as a "set of fixes" elsewhere in this wiki, even though there is no reason to believe this behavior is not intended.)
When cast by a level 40 mage, this spell just edges out Isaac's greater missile storm and hellball for the greatest damage (40d6+10) potentially inflicted on a single target by a spell.