4bn divided by 7500 gives me a bit over 530k. There are that many electric vehicles sold in the US per year?
I asked the same question. The tax credit as it stands now has a per-manufacturer quota of 200,000 units. Once it is reached, the credit is gradually phased out for that manufacturer's products. I guess the (not entirely unreasonable) idea is to help the manufacturer with the startup costs for EV production. Once the NRE is covered, the support stops.
So 4bn appears to be the lifetime gain, not per year. In the grand scheme of the US finances, that's slightly less than nothing.