It may actually not be needed to convert mp4 to 264, considering both files are essentially the same, i.e. MPEG-4 container that contains AVC video data. The only difference is in suffix, so a simple rename from mp4 to 264 should be enough, if you for some reason need these files.
Many multimedia converters will also offer mp4 to 264 conversion, but it will essentially achieve the same thing as renaming the suffix.