This suggests that distributions other than those reportable on Form 8606 line 7 are being included on Form 1040 line 4b, such as pension, 401(k), 403(b), 457(b), federal TSP or annuity distributions, or Roth conversions, reported on Forms 1099-R other than the one being included on Form 8606 line 7. All taxable amounts from all Forms 1099-R go on Form 1040 line 4b.
