Level 4

Animal Summoning I

Type: Summoning
Strategy: Like the mage series of monster-summoning spells, this summons spell animals to fight for the caster. It does, however, summon fairly low hit point creatures that do almost no damage. Though some of the later summon spells are great to cast for creatures that absorb damage, this won't do much of anything and is just a waste of time.
Importance: Low

screenshot
Jaheira summons forth a woodland being.
Call Woodland Beings

Type: Summoning - druid only
Strategy: Call woodland beings summons a nymph to help the party. Though nymphs make fairly lackluster fighters, they have a few spells at their disposal and can heal, confuse, and dominate enemies. If you need more spells, she may be worth summoning, but by the middle of Baldur's Gate II, the nymph's spells do almost nothing. Since she can't fight well, she loses any importance and is quickly not worth casting.
Importance: Low

Cause Serious Wounds

Type: Touch damage
Strategy: Since priests can wear most armor, they can naturally double as fighters. So, though touch damage spells are abysmal for mages, they serve to boost the already competent fighting abilities of the priest. For these spells, you must wait until you are attacking an enemy worth casting this spell on. Then, once you are out of harm's way, and the target is concentrating on attacking another party member, cast this spell. Once cast, you have two rounds to successfully hit the enemy. Once you do, this spell will deal 17 points of damage to it (with no saving throw). If you don't hit it, though, the spell will be wasted. So, make sure your priest has good enough weapons, and you are in a position to hit. All in all, if used correctly, cause serious wounds, like the rest of the line of priest touch spells, is extraordinary.
Importance: High

Cloak of Fear

Type: Hindrance
Strategy: Like the mage spell horror in effect, cloak of fear creates a magical cloak around the caster that will instill fear within a three-foot radius of the caster. This is a nice spell if you are getting overwhelmed by low-level enemies and need time to deal with a few of the enemies at a time, as it will give you time to hit them when they can't hit back. Since the cloak moves with the caster, this spell will also help alleviate the pressure on the front lines. Remember, though, that when the spell wears off, the terrified creatures will be back in action. The monsters get a save, so do not expect them all to be affected. However, there are higher-level fear spells that can take effect on higher-level creatures, get resisted less, and have a longer effect. For the level, though, the spell works acceptably on those lower-level creatures.
Importance: Moderate

Cure Serious Wounds

Type: Cure, heal
Strategy: Cast it on a hurt party member. This spell heals 17 points of damage. For Baldur's Gate II, that amount is acceptable and can be used outside of combat to heal the party back to full strength. However, the spell is also useful to cut down on hours of rest, so if you find no other spells, fill up level 4 with it.
Importance: High

Death Ward

Type: Defense
Strategy: This spell grants the target protection from spells like disintegrate, power word: kill, death spell, and finger of death. Since higher-level mages use these types of necromantic spells often, death ward becomes increasingly needed at higher levels. Once you start seeing these types of spells cast on you, you may want to keep a copy of death ward memorized to protect your most fragile party member. The downfall is that it only affects one creature, so it is impractical to defend the entire party.
Importance: Moderate

Defensive Harmony

Type: Defense
Strategy: Granting a +2 bonus to the armor class of all party members within ten feet of the target, defensive harmony can turn good fighters into untouchable instruments of death. A two-point bonus to armor class should not be underestimated and will help at all times in the game. Since this spell affects an area, one cast will benefit the entire party. Overall, it's very useful in any and all battles.
Importance: High

Farsight

Type: Other
Strategy: This spell is identical to the wizard spell of the same name. Farsight is a spell that often comes in handy, revealing a portion of the map and clearing the fog of war. This will let you easily scope out an area and memorize and cast needed spells before barging in. It's very useful for the more difficult areas of the game.
Importance: High

Free Action

Type: Cure, heal
Strategy: Working in much the same way as dispel magic, free action dispels all movement-hindering spells. That includes those that dispel but will not free, like web and paralysis. Overall, one copy of this spell must be memorized for the times for when these spells are cast on your party. In general, though dispel magic will clear many spells, it will not clear everything. Free action is one of the spells that pick up what's left.
Importance: High