Option 1 states that the Santhals were in a desperate situation as tribal lands were leased out. This implies that the Santhals were facing difficulties due to the leasing out of their lands. Option 1 is correct.

Option 2 states that the Santhal rebels were treated very leniently by British officials. This suggests that the British officials did not punish the Santhal rebels severely. Option 2 is not mentioned in the provided answer, so it is incorrect.

Option 3 states that Santhal inhabited areas eventually constituted into separate administrative units called Santhal parganas. This implies that the Santhal areas were established as separate administrative divisions known as Santhal parganas. Option 3 is correct.

Option 4 states that the Santhal rebellion was the only major rebellion in mid 19th century India. This implies that the Santhal rebellion was the primary rebellion during that time period. Option 4 is correct.

Based on the explanations above, option 4 is the correct answer as it correctly states that statements 1 and 3 are correct.