A man with hemophilia (a recessive, sex-linked condition) has a daughter of normal phenotype. She ...

The man is X*Y where his X-chromosome carries the mutation. He shows the phenotype as he has no back-up good copy of the affected gene.

His daughter inherits the affected X-chromosome from her father and I assume a normal X-chromosome from her mother so she X*X. Since she has a back up good copy of the gene on her unaffected X-chromosome she is phenotypically normal.

She marries a man who is normal, XY.

Any daughters produced from this mating will have at least one normal X (from the father) so will be phenotypically normal i.e. 0% chance of having haemophillia.

Any sons produced from this mating have a 50% chance of inheriting the affected X-chromosome from the mother i.e. 50% chance of haemophillia for any son.

For all 4 sons: (50%)^4 = 6.25%