The Indus Dictionary Project
This is a pictograph of four counting rods. It is the numeral four.
Mohenjo-daro: Seal: M-331 a: Jagat Pati Joshi and Asko Parpola, 1987: Corpus of Indus Seals and Inscriptions: Volume 1: Page 84: Collections in India: Suomalainen Tiedeakatemia.
It is important to distinguish this symbol from the four, full character length, lines (tracks) that depict two adjacent road lanes.
Indus Script Sign Number 1019: List of Sign Variants: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.