IRS refunded is $483. Turbo Tax guarantee refund $3721. what to do now?

TurboTax has a special phone number for help with tax letters. Click on the blue button that says "Get help from TurboTax support" after you click here.

Just in case, How to submit documentation for accuracy guarantee claims.

Is your Social Security 0 on 1040 line 6b?  

What usually happens is the IRS changed some other income on your return. Like the taxable amount of a 1099R you got or you missed entering some income. Like if a 1099R got entered on the wrong line 1040 line 4 or 5, the IRS would miss it and think you didn’t report it.  By increasing your other income it made more of your SS taxable.

Or There is a new question this year asking if you lived in a foreign place. People have been answering it wrong or skipping it. Go back through the Social Security entries.

See this thread that has a screenshot
https://ttlc.intuit.com/community/retirement/discussion/re-tax-on-social-security/01/3613956#M245879