"considering males have only one X chromosome": Around 1 in 500 males have two X chromosomes (XXY), so it should be possible for males to end up as tetrachromat

"considering males have only one X chromosome": Around 1 in 500 males have two X chromosomes (XXY), so it should be possible for males to end up as tetrachromats too (although this wouldn't explain 8%). See https://en.wikipedia.org/wiki/Klinefelter_syndrome https://en.wikipedia.org/wiki/Klinefelter_syndrome