If you received letter number 4883C, it is a legitimate IRS letter number.
Your letter is probably legitimate if is asks you to call 800-830-5084 (the number listed on the IRS website where more information about this letter can be found: https://www.irs.gov/individuals/understanding-your-4883c-letter)
When you call, please have the following information available:
- The 4883C letter that you received
- A copy of your prior year tax return (if you filed one)
- A copy of your most recently filed tax return
- Any supporting documentation (W-2s, 1099s, etc.) for both returns
