Hi
It is a form of martial arts.
Sqay is a South Asian form of sword-fighting originating in Kashmir, currently divided between India, Pakistan and China. Armed sqay makes use of a curved single-edge sword paired with a shield, or one sword in each hand. Unarmed techniques incorporate kicks, punches, locks and chops. (Source: Wikipedia)
I do not think 4 year old is too early. But you can wait for a few more years. My daughter is learning it in school but is the unarmed form, which includes punches, kickboxing, etc.