Social security tax is fixed at 6.2% and Medicare is fixed at 1.45%, there is nothing you can do about it and nothing on your W-4 can change that. If the amounts are more than 6.2% and 1.45%, check with your employer.
Note that Social Security tax is only charged on the first $168,800 of your income. If your salary is higher than that, then once you have been paid that much, the employer will stop collecting FICA for the rest of that year. Medicare keeps going. See this blog post.
https://turbotax.intuit.com/tax-tips/general/fica-and-withholding-everything-you-need-to-know/L8Vjhs...
