Even without the government voucher, downsizing from a Lincoln Navigator to something like a Lexus RX would be worth it for the fuel savings alone.
Someone wanting to stay in the Lincoln family could get the MKX, which gets 1 mpg less. While not as nice, in many ways, the MKX is just as dependable as a Lexus and would still qualify for a $4,500 credit. A buyer could probably negotiate a lower purchase price for it, too.
NEXT: Mazda5 for Dodge Caravan