You don't give enough details of your situation. If your wife is under age 59-1/2, or was under age 55 when she separated from that employer, the 401k withdrawal is subject to regular income tax (which could be up to 32% depending on income) plus a 10% penalty. At very high income levels you might also be 3.9% net investment tax. So 20% withholding might not have been enough.
On the other hand, if you are retired, the extra 401k income won't be penalized, but it might have increased the amount of social security benefit that is considered taxable, and raised your tax more than you expected.
You should first make sure the 20% withholding is recorded in box 4 of the 1099-R and you are getting credit for it on your tax return. Your withholding should also be on form 1040, lines 25a-d.
Assuming your return is correct, there is almost nothing you can do after December 31 of the year that can reduce your income tax. The only things I can think of are to make tax-deductible contributions to an IRA or HSA, but you have to be eligible first. If you are working, your ability to contribute to a deductible IRA may be limited by your income and participating in a workplace plan. If you are retired, you can't contribute to an IRA at all unless you have "compensation" from working (wages or self-employment income). To contribute to an HSA you must be enrolled in a qualifying high deductible health plan and have no other health coverage, such as medicare.
