It is legal to date your fourth cousin. Laws prohibiting relationships between cousins are only applicable to first cousins. Fourth cousins are not close enough genetically to support a charge of incest or an increased risk of birth defects. The reasons for banning marriages among first cousins do not apply.
Laws restricting marriage between first cousins differ greatly between states and countries. It is actually legal to marry your cousin in 26 American states, as well as in many countries around the world. These marriages are banned or regulated in China, India, the Balkans and the other 24 American states.
Many governments passed laws restricting marriage between first cousins to prevent birth defects in children. Children born to parents who are first cousins are more likely to suffer from birth defects than children born to parents who are more distantly related or not related at all.