I think anyone stating they can sing higher than four full octaves is overstating a bit. I think there are some, but it's a very tiny list.
Prime era Tate was a four-octave singer who was a high baritone/low tenor in his early 20s. And many of the classic metal singers could do that crazy range...for a time.
What I always found confusing is people don't understand what "falsetto" is. Hitting a note by narrowing an air passage isn't the same thing as a gifted tenor singer hitting the top end of his or her head voice.
For example, I'm a mid-baritone. I'm not very rangy (Think Scott Stapp, Scott Weiland, 90s era stuff, Mark Tremonti (who is closer to bass), or even Billy Joel or Darius Rucker from Hootie and the Blowfish). But if I have to sing the stuff I love (which is difficult, because a lot of it is in a key too high for me to sing in), if I go for a high note in a song that's in a higher key than I can sing with my head voice, I have a strong enough falsetto to hit the note. My personal issue is making sure that I can control the way my falsetto sounds so it isn't off-key while doing that, instead of just an annoying high pitched note.
And most of the time, I fail at it.
Thus, me sticking with Pearl Jam, Ozzy, Creed, etc.
Rob Halford is very interesting, because he would sing in his head voices and hit those notes for years. And about 25 years ago, he discovered his falsetto was very strong, and has been using that to hit high notes for years. Not many folks can do that. Tate tried falsetto in mid-2003, before the DT/QR/FW tour, when the band was over in Europe. Didn't quite get there. But here we are in 2023, and what is he doing? He's finally using his falsetto, because he's got it now where he can hit notes with it, since he can't get up into his head voice like he used to.
But not everyone can figure out how to use their falsetto. And thankfully, many great singers don't have to use it (it's actually the wrong way to sing). Corey Glover of Living Colour is a baritone, but sings notes up and down in his head voice that will blow you out of the water. He is a true four-octave singer. Same with Myles Kenned. Four octaves - all head voice. No falsetto. At least not yet.
I'm not a professionally trained singer. My wife is (who does NOT use falsetto). But vocal range...the way it is explained to me, is that it is a combination of the how low to how high you can go in your natural voice (chest, mixed, head), and sing that comfortably, on key. Falsetto does not "count" per se. At least not in the way I understand it.