For whatever it’s worth, the OED has four-door for the adjective. They provide this citation:
- 1957 P. Frank Seven Days to Never i. 13
A four-door sedan.
So probably that’s what you should say unless you mean something
else than is meant by the way their citation uses it. See below for
such contrasting examples using other pairs than yours.
What you write is of course up to you. These normally are hyphenated
in writing when used attributively, but not always: there are still a
lot of five pound notes in circulation.
Usually you have only one of either "a № NOUN something" versus
"a № NOUNed something" in common use, but which it is depends on other factors.
For one thing, measure phrases used attributively seldom inflect their
nouns for number the way they often do when used predicatively. So you
have things like:
- a twenty-one-gun salute
- a five-acre farm
- a five-day week
- the four-colour problem
- a four-letter word
- running a four-minute mile
- a four-stroke engine
- the million-dollar question
Compare those with “more adjectival” NOUNed versions:
- a three-legged stool
- a five-fingered jack
- a game of four-handed cribbage
- some five-leaved grass
- a two-sepalled blossom
- that hundred-headed thistle
Nobody today says ❌ a five-dayed week because normally such measure
phrases used attributively start out with an uninflected noun, so you only
have a four-day week. But we used to say ❌ a four-inched whatever,
even though we now say only a four-inch whatever.
Sometimes both versions exist. If so, they might mean the same
thing the way they do in one-finger salute and one-fingered salute.
But they might also mean different things as they do in a three-foot dog versus
a three-footed dog, or a five-point essay versus a five-pointed
It really just depends — there are incalculably many more such example
pairs out there, both contrasting ones and non-contrasting ones.