[Test Name][Year][Q#]- Biology genetic question

The color of the fur of dwarf rabbits is determined by a single gene with four alleles of which only one is dominant; only in the presence of this allele is the hair color dark grey. What is the relationship between the genotypes that determine the dark gray color of the fur and the possible genotype combinations?
A)4/10
B)3/10
C)4/7
D)3/7
E)3/4
The correct answer is A. I tried at the first time to see how many options we have for dark grey color. since we need at least one dominant allelle present for coding dark grey color, then the number of options that we have is 4.
From this point I dont know how to proceed.
Thank you all for helping in advance

Very interesting I had to ask just to make sure, this should be the law of independent assortment correct?