The nature of religion according to the Rig Veda can be described by the characteristics mentioned in options 1, 2, and 4.

Option 1 states that the Rig Vedic religion can be described as naturalistic polytheism. This means that the religion recognizes multiple deities or gods, and these gods are often associated with natural elements and forces.

Option 2 states that there are striking similarities between the Rig Vedic religion and the ideas in the Iranian Avesta. The Iranian Avesta is a religious text of Zoroastrianism, and the similarities between these two religions suggest that there may have been interactions or influences between them.

Option 4 states that Vedic sacrifices were of two kinds - those conducted by the householder and those that required ritual specialists. This indicates that the Rig Vedic religion involved ritual sacrifices, and these sacrifices were carried out by both ordinary people and specialized priests.

Option 3, which states that Vedic sacrifices were conducted in the house of the priest called yajaman, is not an accurate characteristic of the nature of religion according to the Rig Veda.

Therefore, the correct answer is option 4.