i think it may be 4n… here’s why
the tetraploid plant’s pollen grain would then be diploid, no?
so 2n combines with the polar nuclei… 2n+n+n= 4n
normally endosperm are triploid, and i thought they always were… but apparently that’s not always the case. if you haven’t, look here http://en.wikipedia.org/wiki/Endosperm