The first digit of all Visa credit card numbers is 4.
That opening digit identifies the Visa payment network within the card-number system defined by ISO/IEC 7812. The following digits identify the issuing institution and account range, while the final digit is commonly a check digit used to detect typing or transmission errors.
Visa numbers are not all the same length: traditional Visa card numbers may contain 13 or 16 digits, and some modern Visa numbers contain 19. The length can vary, but the leading 4 remains the familiar network identifier. This is why many online checkout forms can recognize a Visa card after only the first digit or two.
A common mix-up is confusing the first digit with the first six or eight digits. Those longer prefixes identify an issuer range, whereas 4 is the broad Visa identifier. Other major networks historically used different starting ranges, although Mastercard expanded beyond its older 5-series range in 2016.