Code 4 indicates that this was a distribution from an inherited traditional IRA, not from an inherited Roth IRA, and is therefore taxable. Unless your mother had basis in nondeductible traditional IRA contributions, the entire amount is taxable and you likely owe the taxes and interest being assessed by the IRS. Had you entered this into TurboTax, the taxable amount would have been included on Form 1040 line 15b.
Although unlikely, it's possible that the payer prepared the Form 1099-R incorrectly. You would need to take that up with the payer.
