Tax Withholding

I received a letter from the IRS on November 13 questioning the amount listed in my return on Line 17, page 2 of my 1040, Federal Income Tax Withheld from forms W-2 and 1099.  The line has $280,503.  When I went back to my W-2's they show $270,046 for me and $5,867 for my spouse, total $275,913.  I don't recall entering any tax withholding from a 1099 in turbo tax.  Can you point out to me where the additional $4,590 might be so I can show IRS?  Please feel free to call me at [phone number removed] to discuss.  Thank you.