concur -- I have large-firm experience in this area; for large firms, they generally will not pursue bonus repayment very vigorously from a departing employee -- not worth the firm's while to pay an attorney a couple thousand $$, and potentially get a bad name, in order to try to get repaid. If the firm did something bad/unethical and you decided to leave despite the contract that says you must repay the 10K... some people may elect to repay, others may not. It is very unlikely that the firm would turn you down if you offered to repay 8K immediately rather than 10K, possibly, at some point way down the road.
Note 2 -- the 4K paid toward taxes is not really relevant, other than as it impacts your short-term cash liquidity -- there are tax requirements regarding bonuses and federal withholding tax on bonuses paid, but if you are paid 10K and then have 4K of taxes withheld (this counts as 10K income), and then have to return the 10K (returning the 10K will reduce your taxable income by 10K), you will not be responsible for the 4K taxes that were withheld, as there is no longer a net increase in taxable income of 10K because the full amount was returned. You'll get the full 4K back at the end of the year when you file taxes. Alternately, you can increase your deductions for the rest of the year so that less is taken out; the benefit of this is that if you can reduce taxes paid by, say, $500 per month for 6 months, instead of waiting until you file taxes to get the $4K back, you've already gotten $500 * 6 or $3,000 in your pocket due to lower taxes paid in the current year, and will get the other $1,000 back when you file taxes the next year.
