Per your FICO score, you might see as much as 100 points lost with the initial lates, repo, and charge-off. You'll see additional points lost if/when a CA and PR report. It all depends on your overall history.
Going off on a slight tangent....Don't Do It!!
If being upside-down is the determinant, then you'll have to know that everybody is upside-down when they purchase a car. A car doesn't appreciate in value (unless you bought an exotic) when you buy it. When you drive off the lot for the first time, the car is only worth a fraction of what it once was and unless you put down a large down payment to cover that depreciation, you're already upside-down. And I know it gets discouraging when you look at the KBB value and compare that to your loan statement. That can be cured by paying more on the principle, and eventually you won't be upside-down.
Assuming you do turn it in, you still have to make the payments. If you do a voluntary repo, a repo will report right away (voluntary, involuntary it doesn't matter). The car will then go to auction and you are liable for the amount of the loan MINUS the amount they auction it for plus any fees like added interest, fees from the lender, towing fees from the dealer and so forth.
If you don't PIF by that point or work out a payment arrangement, you then will get hit with a 30 day late, 60, 90, 120 and so on all the while adding interest and fees. Then it'll get charged-off and sent to a collection agency. The collection agency will report too. At some point either the CA or the OC can, if they so choose to, sue you for the amount of the loan plus the difference in the loan value and what they sold it for plus interest plus fees. If they win, then there's garnishments and so on to deal with.
The $4k deficiency shouldn't be a concern. Just keep paying it. In fact if you look at any amortization table, you'll see that towards the end of the loan nearly all of the money goes towards the principle. When you get the statements from the lender you'll be amazed how fast that balance drops.
Now if paying it is an issue, then tighten your belt. Cut back expenses and increase income. Throw whatever extra you have to pay the thing off. If you let it get repoed, a $4k deficiency would be the least of your worries.
