Tolkappiyam is an ancient Tamil grammar text written by Tolkappiyar, hence option-1 is correct as it is indeed a treatise on grammar. Option-2 is also correct because it has been dated to between the first and fourth centuries CE. Option-3 is considered accurate, it is believed to be a part of the Sangam period`s extensive body of literature, thus making it a part of the Sangam corpus.

The incorrect statement here is option-4. Tolkappiyam was not composed by Panini. Panini is known for his work on Sanskrit grammar, not Tamil. He wrote "Ashtadhyayi", a Sanskrit grammar guide, and had no role in the creation of Tolkappiyam. So, the correct answer is Option 4.