Alas, I've had to remove the Big Four of Technical Death Metal and the Big Four of German Thrash due the abysmal number of correct answers. I may add them to a sequel if this one gains traction.
Where did the "Big Four of Power Metal" come from? because I've definitely heard of the big four of Thrash and of Grunge - those lists exist not just because those four were big, but because they were instrumental in defining their respective genres. I've never heard of a big four of Power Metal, but if I was in charge of making a list for one, I definitely would never include Dragonforce and Sabaton. I love them both, but they came around long after the genre had already been established. I would have put Gamma Ray and Manowar, or maybe Stratovarius.
Good question. It was mentioned (with references) on all 4 bands Wikipedia pages. I've now checked the references, and they all link to a 2021 article from Loudwire by Jordan Blum, The ‘Big 4′ Bands of 17 Metal Subgenres. It's purely an opinion piece by Loudwire though, and isn't widely accepted at all, so I'll look to change those questions in the future. The article has some really controversial picks and I remember deliberately avoiding it when I wrote the quiz. Thought Wikipedia was a tad more reliable.