Answer
50%
Work Step by Step
This is because the pedigree shows that the affected females are heterozygous (each has an unaffected parent who would give her a recessive allele since this is a dominant trait), marrying an unaffected (homozygous recessive male), means that the fourth generation would have the same chance of being affected as of getting a dominant allele from the affected mother, 50% since each of those females is heterozygous.
