Remember there are two separate tax issues regarding social security.
A. Before your full retirement age, too much earned income will reduce your benefit.
B. At any age, your other taxable income will influence the amount of income tax you pay on your social security benefit.
A distribution from your 401(k) is taxable income, and may increase the amount of tax you pay on your social security benefit. But it is not earned income and will not reduce your benefit if you declare retirement before full retirement age.
