Two 18" subs in a sedan boot may work, depending on the enclosure design. Four I just can't see happening.
No real point in removing back seat because it would be illegal on the road without reregistering it as a two seater, and I believe it would only be allowed in the unrestricted class at SPL comps, but would have to ask someone who has been in comps to check that. Usually in the unrestricted class you will be going up against people with WAY to much money.
-Democracy must be something more than two wolves and a sheep voting on what to have for dinner.
James Bovard, Civil Libertarian (1994)
-Giving money and power to government is like giving whiskey and car keys to teenage boys.
P.J. O'Rourke, Civil Libertarian
-Government's view of the economy could be summed up in a few short Phrases: If it moves, tax it. If it keeps moving, regulate it. And if it stops moving, subsidize it.
Ronald Reagan (1986)