Usually the magic number is 3 not 4.
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The main reason it's 4 is because of the Damage/Time/MP/TP ratio. At 3 enemies the damage and mp/tp usage usually evens out with the time maybe being a bit more than focusing enemies down one by one. 4 gives you the best advantage to using AoEs over single target skills. 3 is the middle ground where it doesn't matter and could be up to the players preference. But of course there are exceptions and variences, like Samurais Iaijutsu, Ninjas Doton/Katon and Frog, Paladins Circle of Scorn, Bards Rain of Death, etc, from a strictly AoE point.