I am pretty sure if you and the seller have an agreement to sell the car for $4,000, then everything is good once you trade the silver off. Of course, I think if the seller goes to cash out the silver, they might run into some problems with theIRS. I am no professional, and I highly recommend you talk with a tax attorney on this one. Let me know how that works out.
ITS LEGAL!
The constitution says that the govt. has to coin money out of gold and silver, so here's a situation. I get my yearly salary in face value 50$ gold pieces, its too low to tax anyways, but according to the constitution there in nothing wrong because it is official currency with the face value of 50$ and that means its perfectly legal