I highly doubt that Nintendo will have UHD blu ray. The BOM of a UHD blu ray drive must be in hundreds of dollars. Furthermore, due to nature of the HDMI spec a lot of 4K TVs will not be make use of all of the features of the UHD blu rays.
You can literally buy a 4k capable lg blu ray burner for $40-$50 at retail, so BOM is probably well under $20.