Number of cells participating during double fertilization are
Double fertilization = Syngamy + Triple fusion
During syngamy, one male gamete cell (n) will fuse with egg cell (n) to form zygote.
During triple fusion, one male gamete (n) fuse with two polar nuclei (n+n) in central cell to form primary endosperm cell (3n; PEC). Therefore, in all four cells participates in double fertilization.