To determine which statements are incorrect about the Jhumur dance and related cultural aspects, let's evaluate each statement given:

1. Statement (i): The tea garden community migrated from Central India in the 19th century to work in the tea gardens of Assam.
   This statement is correct. Historical data shows that the workforce for Assam's tea plantations was indeed brought from Central India during the 19th century.
2. Statement (ii): Jhumur dance originates from the Sadan ethnolinguistic group of the Chota Nagpur region.
   This statement is correct. The Jhumur dance has its roots among the Sadan ethnolinguistic group of the Chota Nagpur region.
3. Statement (iii): The songs sung during Jhumur performances often depict the struggles of tea plantation workers. They narrate stories of migration and exploitation, depicting the community's socio-economic challenges.
   This statement is correct. The essence of Jhumur songs often revolves around the historical and socio-economic narratives of tea plantation workers, including tales of migration and hardship.
4. Statement (iv): Jhumur dance plays a vital role in tea garden festivals, particularly during Tushu Puja and Karam Puja, which celebrate the harvest time.
   This statement is correct. Jhumur dance is an integral part of cultural festivities like Tushu Puja and Karam Puja in tea garden communities.

Since all the statements accurately describe aspects related to the Jhumur dance and the community involved, the answer is that none of the above statements are incorrect.