They are both heterozygous for the trait.
They both express the dominant trait, but one of the offspring is recessive. The only option for this is that both are heterozygous and have two genes: recessive and dominant.
You can see this when you draw a Punnett square.
Subject 3 will be Dd where D is the dominant trait, and d is the recessive trait. Subject 4 will be Dd.
Now cross them, and you'll found 25% chances of having a recessive offspring.
You can't have only one of them heterozygous because, when you draw the Punnett square, you'll have 50% dominant and 50% dominant heterozygous.
The same happened with only one homozygous.
And, if they were both homozygous dominant you only have one trail, no two, and again you can't have a recessive offspring.
