hi , i m on H4 visa, and i want to do my own business (kiosk in mall) . is it legal to do that ?

Unfortunately, it isn't legal - not on the H-1B anyway.

There are circumstances where you can open your own business in the U.S. There are visas for investment - the E-1 Treaty Trader and E-2 Treaty Investor - but as the names imply they require a treaty to be in place between the U.S. and your home country; we don't have these agreements for every country. Also, the E visas have fairly specific, and rather stringent requirements - it isn't clear to me that a mall kiosk would be a large enough business/investment to use for E-2 treaty investor purposes, and it isn't a trade (import/export) business.

There are cases where one can get an H-1B to work in a professional capacity even with one's own business, but it isn't clear that this would work here either.

Speak with an immigration attorney directly to see what options might be available to do in order to pursue this. Good luck.