In the Japanese-language edition of Metal Gear Solid 4: Guns of the Patroits, Big Boss is voiced by Ōtsuka Chikao, the real-life father* of Ōtsuka Akio, who voices Old Snake (and has been voicing Solid Snake in the Japanese-language editions Metal Gear series since MSG1 on the PlayStation, and also voiced Naked Snake/Big Boss in the Japanese-language editions of MGS3, MGS:PW and MGS5:GZ**).
*I don’t know any relation between Richard Doyle (the English voice of Big Boss in MGS4) and David Hayter (Old Snake’s English-language voice actor). though. If anyone does know, please tell me.
**At least Japanese fans still have its Snake—the English-speaking fans had mixed reactions to Kiefer Sutherland replacing David Hayter as Big Boss in Ground Zeroes!