The word actually comes from the B2/Type 81 sedan Audi sold from 1980 to 1987. It was a four door sports sedan that was offered with quattro all-wheel drive as well as regular front wheel drive. It sold quite well and led the way for the Audi 80/90, and then the Audi A4/S4. It was a comprable car to the BMW 3-series from the same era.

It is now used as an expression meaning to leave a location. It evolved from "we are outta here", to "we are outtie", to "we are audi" as the t's were slurred into d's. To put an original spin (in light of the resurgence of the 1980's nostalgia), "4000" or "5000" was added in light reference to the quintissential 1980's yuppie mobile that was the Audi 4000 and 5000 (the 5000 was slightly larger and more powerful than the 4000).
We are audi 4000!
by Solo03 May 4, 2004
total piece of shit that only people with small dicks drive
"yo look at the piece of shit driving down the highway"
"damn man that must be one of those audi 4000's"
by anthony arena May 15, 2007