although... who's to say only the four of them or the CHS students with Quirks? might there be another?

Is it twilight.

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Question will you still continue W. I. T. C. H story

After the Sirens had been defeated and Twilight and I brought the villain with us back to Equestria, we’d gotten a surprise, in that he didn’t turn into a pony when he passed to the other side.

What a disappointment.
The sirens come to CHS to feed of high nutrient and easy targets. Risks free. If they didn't change their plan after this, then it only mean author doen't have enough guts to write story without using FiM's plotline directly