You enter the 1099-R for the 401(k) distribution. Assuming that this is a direct rollover/conversion by the 401(k) trustee, the 1099-R box 1 and 2a amounts will be the same. There will be no tax withheld on box 4 so you do not enter the tax anywhere. The tax paid from another source would be entered as an estimated tax in the estimated tax section and have nothing to do with the 1099-R.
(Note: if this is not a direct trustee transfer then the trustee is required by law to withhold 20% for tax.)
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
