This is according to Wikipedia. The four platforms are, in descending order of length, at Gorakhpur (Uttar Pradesh), Kollam Junction (Kerala), Kharagpur Junction (West Bengal) and Pilibhit Junction (Uttar Pradesh). The longest of all, at Gorakhpur, is 1,366.33 metres (4,483 feet) long; the fourth longest, at Pilibhit Junction, is 900 metres (2,953 feet).
The longest platform outside India is at the Auto Club Speedway in Fontana, California; it's 815 metres (2,675 feet) long.
The sixth longest is in India, and the seventh is in England: at Cheriton Shuttle Terminal, in Kent. It's 791 metres (2,595 feet) in length.
© Haydn Thompson 2021