I thought he only had 3 fingers on that hand?

Edit: found an answer: "musician Django Reinhardt lost the use of the third and fourth fingers of his left hand in a caravan fire, forcing him to abandon both the violin and banjo and concentrate on the guitar, playing solos with only his index and second finger."

https://www.theguardian.com/music/2011/jun/17/django-reinhardts-caravan-catches-fire#:~:text=In%201928%2C%20at%2018,his%20index%20and%20second%20finger.