Quote from KaiserXavier >>

This should read:

You tap into the nightmares of a creature you can see within range and create an illusory manifestation of its deepest fears, visible only to that creature. The target must make a Wisdom saving throw. On a failed save, the target becomes frightened for the duration and at the end of each of the target's turns before the spell ends, the target must succeed on a Wisdom saving throw or take 4d10 psychic damage. On a successful save, the spell immediately ends.

At Higher Levels. When you cast this spell using a spell slot of 5th level or higher, the damage increases by 1d10 for each slot level above 4th.

Exactly. This makes the most sense out of all the comments here. If you're not frightened, taking the damage would make no logical sense. Just like on the spell Vicious Mockery, why would you have disadvantage on the next attack roll if you DIDN'T take the 1d4 psychic damage?! 

As a logical DM myself who is going to use this spell against players, if they succeed on the frightened condition, the entire spell ends. Don't like it? Kick rocks ya noobs! XD