The 2021 Form 1099-R with codes 8 and 4 in box 7 should show in box 2a only the gains attributable to the amount being returned. Only this amount is taxable and only this amount will be included on Form 1040 line 4b.
If you were refunded $21k, how is it that you received only $18k? Did they make a return of contribution of $21k but due to investment losses the loss-adjusted amount distributed was only $18k? If so, the Form 1099-R should have a zero in box 2a. If the amount in box 2a is equal to the amount in box 1, you should request a corrected Form 1099-R. Box 2a equaling box 1 on a code 8 Form 1099-R is nonsensical.
