How can i get him to behave better?!?

okay so im 13. Im the oldest and i have a little sister who is 12. and i have a a 4 year old little brother. Sometimes hes good and sometimes hes bad. Its mostly when we areound other people or in public hes bad. We have tried everything!!!!!! Like when we are in the mall.......or walmart if he doesnt get what he wants he screams and lays on the florr for a temper tantrum.He going to be 5 in august. me ad my grandma thinks he is fighting for attention. but even when we give him attention hes till bad. Its embarrasing he way he acts. hees out of control . its rideculous. so we him. we sit him in timeout. we ground him from the nintendo and tv. and we started a a good behavior sticker chart. and that worked for a while but now hes not amused with it. we have tried everything!!!!!!!!!!!!i mean out whole family gets pushed around by him. and it needs to stop. so can someone give me some tips to tell my grandma......... so we can stighten him out casue if he acts like this during..... elementry school. hes going to be in a lot of trouble.