Rebate?

Or credit?

The EV tax credit is a nonrefundable tax credit offered to taxpayers who purchase qualifying electric vehicles or plug-in hybrid vehicles. Nonrefundable tax credits lower your tax liability by the corresponding credit amount but do not result in a refund of any excess credit amount

Credit. Sorry for the confusion 

There may be a loophole which is that if you transfer the tax credit to a participating dealer, then the dealer can lower the cost of the vehicle up to the 4k. Am not sure how this applies to my client's situation

That is done at the time of purchase. It would be shown on the purchase receipt.

Too late now.