Contradiction with Genesis 4:2
Both Abel and Jabal are noted as keepers of sheep, whereas Genesis 4:20 only mentions Jabal.
Genesis 4:2: And she again bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground. [Abel: Heb. Hebel] [a keeper: Heb. a feeder]
Contradiction with Genesis 4:17
Cain builds a city, indicating settled life, which contrasts with the nomadic lifestyle implied in Genesis 4:20.
Genesis 4:17: And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch. [Enoch: Heb. Chanoch]
Contradiction with Ecclesiastes 1:9
Suggests that nothing is new under the sun, contrasting with Genesis 4:20 introducing livestock tending.
Ecclesiastes 1:9: The thing that hath been, it [is that] which shall be; and that which is done [is] that which shall be done: and [there is] no new [thing] under the sun.
Contradiction with Genesis 1:24-26
God creates animals and humans simultaneously, whereas Genesis 4:20 implies a progression or development in animal husbandry.
Genesis 1:24-26: And God said, Let the earth bring forth the living creature after his kind, cattle, and creeping thing, and beast of the earth after his kind: and it was so.