26 C.F.R. § 20.2014-6
The credit for foreign death taxes under section 2014 is limited to those taxes which were actually paid and for which a credit was claimed within four years after the filing of the estate tax return for the decedent's estate. If, however, a petition has been filed with the Tax Court of the United States for the redetermination of a deficiency within the time prescribed in section 6213(a), the credit is limited to those taxes which were actually paid and for which a credit was claimed within four years after the filing of the return, or before the expiration of 60 days after the decision of the Tax Court becomes final, whichever period is the last to expire. Similarly, if an extension of time has been granted under section 6161 for payment of the tax shown on the return, or of a deficiency, the credit is limited to those taxes which were actually paid and for which a credit was claimed within four years after the filing of the return, or before the date of the expiration of the period of the extension, whichever period is the last to expire. See section 2015 for the applicable period of limitations for credit for foreign death taxes on reversionary or remainder interests if an election is made under section 6163(a) to postpone payment of the estate tax attributable to reversionary or remainder interests. If a claim for refund based on the credit for foreign death taxes is filed within the applicable period described in this section, a refund may be made despite the general limitation provisions of sections 6511 and 6512. Any refund based on the credit for foreign death taxes shall be made without interest.
26 C.F.R. § 20.2014-6