Which statement supports the argument that the coevolution of feedbacks between living things and Earth's systems occurred during the Carboniferous Period?

The Carboniferous Period (approximately 359 to 299 million years ago) was characterized by vast, dense forests of primitive plants. These plants performed extensive photosynthesis, absorbing large amounts of carbon dioxide from the atmosphere and releasing oxygen. The incomplete decomposition of these plants led to the formation of massive coal deposits. Coevolutionary feedbacks refer to how living organisms (biosphere) and Earth's non-living systems (atmosphere, geosphere) influenced each other's evolution.

• (1) As plants died and decomposed, they provided more fossil fuels, which allowed even more plants to grow. While plants did form fossil fuels, fossil fuel formation sequesters carbon, removing it from the active carbon cycle. This process does not directly "allow even more plants to grow" in a positive feedback loop during the period itself.
• (2) The increase in plant life released more water from photosynthesis, which allowed for the development of land animals. Photosynthesis consumes water, and while plants transpire, the primary gas released by photosynthesis is oxygen, not water in a way that directly drives land animal development.
• (3) As less carbon was captured by plants, animals began to develop due to less competition for atmospheric carbon. This statement is incorrect. During the Carboniferous, more carbon was captured by plants, leading to a decrease in atmospheric carbon dioxide. Animals do not compete for atmospheric carbon.
• (4) Plants were releasing more gases through photosynthesis, which allowed for the development of more complex animal life. Photosynthesis releases oxygen ($\text{O}_2$). The immense plant growth during the Carboniferous led to a significant increase in atmospheric oxygen levels. This higher oxygen concentration is believed to have been a crucial factor in the evolution of larger and more complex animal life, as it supported higher metabolic rates. This is a clear example of a coevolutionary feedback between the biosphere (plants) and the atmosphere (oxygen levels), impacting the evolution of other life forms (animals).

Statement (4) accurately describes a significant coevolutionary feedback during the Carboniferous Period, where plant activity dramatically altered the atmosphere, which in turn facilitated the development of more complex animal life.

The final answer is $\text{4}$.