How to write 100% phenotype ratio?
The phenotype ratio would be 4:0.

You did not include the complete example including the genotypes, but an example that might produce this result is:

P is a dominant allele that produces purple flowers, and pp homozygotes are white.

    P    p
  -----------
P | PP | Pp |
  -----------
P | PP | Pp |
  -----------


The genotype ratio is 2:2:0 (2 PP : 2 Pp : 0 pp) meaning there are 50% PP and 50% Pp.

The phenotype ratio is 4:0 (4 purple : 0 white) meaning there are 100% purple and 0% white flowers.

To keep things simple, you do not need to reduce the ratio to the lowest terms (in other words, the sum of the numbers in the ratio should equal the number of boxes in the Punnett Square). It would not, however, be incorrect to reduce the genotype ratio to 1:1:0 and the phenotype ratio to 1:0.