BTW a brand new Civic Si starts at $22K, a used 07/08 is about 10-15k, I don't know where you arrived at the $4k number.
The price quoted does not include taxes. The taxes are paid by the customer at the time of the sale. The tax rates changes from one county to the other (compare county to our praadeshiya saba areas). It's usually around 5-10%, where I live it's 6.75% This tax has to be paid every time a car is bought. Even if I buy a second hand car from a private party I would pay taxes to the state depending on the sale price. There is also a $1500 gas guzzler tax that has to be paid for a brand new car if it does not meet a certain fuel consumption level.