Since there's already a big four of thrash, I'm wondering what other genres you think could have a big four. You could add any genre you like. Here's my picks.
Heavy metal
Black Sabbath
Judas Priest
Motorhead
Iron Maiden (with Bruce/Blaze)
NWOBHM
Iron Maiden (with Paul)
Saxon
Diamond Head
Raven
Speed metal
Exciter
Anvil
Accept
Running Wild
Thrash metal part 2
Exodus
Testament
Overkill
Kreator
Doom metal
Candlemass
Trouble
Pentagram
Saint Vitus
Funeral doom
Evoken
Ahab
Skeptisism
Mournful Congregation
Death metal
Death
Obituary
Morbid Angel
Deicide
Brutal death metal
Suffocation
Devourment
Disgorge
Dying Fetus
Melodic death metal
Hypocrisy
Amon Amarth
Dark Tranquility
At the Gates
Technical death metal
Origin
Necrophagist
Cryptopsy
Nile
Black metal (First wave)
Bathory
Venom
Hellhammer
Celtic Frost
Black metal (Second wave)
Mayhem
Gorgoroth
Immortal
Darkthrone
Grindcore
Napalm Death
Terrorizer
Repulsion
Brutal Truth
Goregrind
Carcass
Last Days of Humanity
Regurgitate
Dead Infection
Gothic Metal
Paradise Lost
My Dying Bride
Anathema
Type O Negative
Progressive metal
Dream Theater
Fates Warning
Watchtower
Queensryche
Power metal
Hammerfall
Helloween
Blind Guardian
Manowar
Symphonic metal
Nightwish
Haggard
Epica
Therion
Folk metal
Skyclad
Finntroll
Enisferum
Korpiklanni
Medieval metal
Corvus Corax
Subway to Sally
In Extremo
Schandmaul
NWOAHM
Avenged Sevenfold
Devildriver
Chimaira
Mastadon
Metalcore
Hatebreed
Converge
Integrity
Rorschach
Melodic metalcore
All that Remains
Killswitch Engage
As I Lay Dying
Trivium
Industrial metal
Ministry
Godflesh
KMFDM
Fear Factory
Groove metal
Pantera
Sepultura
Gojira
Machine Head
Sludge metal
Eyehategod
Crowbar
Acid Bath
Grief
Stoner metal
Electric Wizard
Bongzilla
Weedeater
High on Fire
Crossover thrash
Suicidal Tendencies
Stormtroopers of Death
Dirty Rotten Imbeciles
Corrosion of Conformity
NWOTHM
White Wizzard
Cauldron
Enforcer
Holy Grail
Heavy metal
Black Sabbath
Judas Priest
Motorhead
Iron Maiden (with Bruce/Blaze)
NWOBHM
Iron Maiden (with Paul)
Saxon
Diamond Head
Raven
Speed metal
Exciter
Anvil
Accept
Running Wild
Thrash metal part 2
Exodus
Testament
Overkill
Kreator
Doom metal
Candlemass
Trouble
Pentagram
Saint Vitus
Funeral doom
Evoken
Ahab
Skeptisism
Mournful Congregation
Death metal
Death
Obituary
Morbid Angel
Deicide
Brutal death metal
Suffocation
Devourment
Disgorge
Dying Fetus
Melodic death metal
Hypocrisy
Amon Amarth
Dark Tranquility
At the Gates
Technical death metal
Origin
Necrophagist
Cryptopsy
Nile
Black metal (First wave)
Bathory
Venom
Hellhammer
Celtic Frost
Black metal (Second wave)
Mayhem
Gorgoroth
Immortal
Darkthrone
Grindcore
Napalm Death
Terrorizer
Repulsion
Brutal Truth
Goregrind
Carcass
Last Days of Humanity
Regurgitate
Dead Infection
Gothic Metal
Paradise Lost
My Dying Bride
Anathema
Type O Negative
Progressive metal
Dream Theater
Fates Warning
Watchtower
Queensryche
Power metal
Hammerfall
Helloween
Blind Guardian
Manowar
Symphonic metal
Nightwish
Haggard
Epica
Therion
Folk metal
Skyclad
Finntroll
Enisferum
Korpiklanni
Medieval metal
Corvus Corax
Subway to Sally
In Extremo
Schandmaul
NWOAHM
Avenged Sevenfold
Devildriver
Chimaira
Mastadon
Metalcore
Hatebreed
Converge
Integrity
Rorschach
Melodic metalcore
All that Remains
Killswitch Engage
As I Lay Dying
Trivium
Industrial metal
Ministry
Godflesh
KMFDM
Fear Factory
Groove metal
Pantera
Sepultura
Gojira
Machine Head
Sludge metal
Eyehategod
Crowbar
Acid Bath
Grief
Stoner metal
Electric Wizard
Bongzilla
Weedeater
High on Fire
Crossover thrash
Suicidal Tendencies
Stormtroopers of Death
Dirty Rotten Imbeciles
Corrosion of Conformity
NWOTHM
White Wizzard
Cauldron
Enforcer
Holy Grail