Gross soc sec benefits are 55,064. Medicare premiums total 9660. That nets to 45404. Taxable amount on line 4b shows 46804. Where does the 1400 come from?

Medicare premiums paid are not a direct reduction in income, but an itemized medical deduction taken on Schedule A.  They only reduce taxable income to the extent they exceed 7.5% of your AGI and you otherwise itemize.  In your case, 85% of your social security benefits are taxable, which is the $46804.