The W-4 is used to allow a taxpayer to avoid having to pay the IRS for not having paid in enough taxes during the year. You can account for other credits you receive during the year by adjusting your W-4 accordingly. The Social Security amount received maybe subject to taxation depending on the overall income received for the year. Social Security is means tested and the W-4 will have no effect on that issue. 

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Are there any deductions that qualify to reduce your medicare income?

Medicare uses the modified adjusted gross income reported on your IRS tax return from 2 years ago. This is the most recent tax return information provided to Social Security by the IRS.   The standard Part B premium amount in 2022 is $170.10. Most people pay the standard Part B premium amount. If your modified adjusted gross income as reported on your IRS tax return from 2 years ago is above a certain amount, you'll pay the standard premium amount and an Income Related Monthly Adjustment Amount (IRMAA). IRMAA is an extra charge added to your premium.  Increasing contributions to an IRA or student loan interest deduction can lower your MAGI.  Otherwise, it would primarily be reductions in reportable income.

Below is a useful link with more info:

https://www.medicare.gov/your-medicare-costs/part-b-costs#:~:text=Medicare%20uses%20the%20modified%2....

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