What about many black metal songs that have 4 chords (Burzum springs to mind, and others).
What makes them better, then?
The point is -- oh fuck, is anyone paying attention to what's written on the site?
Rock is composed harmonically. You take four chords that occupy important positions harmonically, like I-IV-V, and wrap a vocal melody around them.
Metal is through-composed and thus composed melodically. that's the Narrative composition ANUS rambles on about. Riffs talk to each other like a dialogue, throughout a piece, and thus they evolve each other like motifs in classical music. They change over time because they have to because melodic context changes. In metal, melodic context replaces harmonic context in rock. Most idiots assume that's just because we're using power chords.
Burzum's four chords are not going to resemble the four chords in rock which are often implied more than expressed. Usually, they're a "modal stripe" as ANUS calls it, a particular set of intervals that convey a mood (remember Plato talking about how all music should be in Phrygian mode?). They work together to form melodies. The harmonic context of metal is either nonexistent or very basic like Slayer's A->B->A dicking around on SOH. The melodic context is much, much more complex.
Tell me, could you harmonize "My Journey to the Stars" to three chords like they did with those tunes?