In the case of double fertilization, the zygote and the endosperm nucleus of a diploid plant would have different sets of chromosomes. The zygote, which is formed by the fusion of a haploid egg cell and a haploid sperm cell, would have 2 sets of chromosomes, represented as 2N. This is because each parent contributes one set of chromosomes, resulting in a total of two sets.

On the other hand, the endosperm nucleus, which is formed by the fusion of a haploid central cell and a haploid sperm cell, would have 3 sets of chromosomes, represented as 3N. This is because the central cell, which already has two sets of chromosomes, fuses with a sperm cell that contributes an additional set.

Therefore, option 4, which states that the zygote and the endosperm nucleus of a diploid plant would have 2N and 3N sets of chromosomes respectively, is the correct answer.