Need help hyundai elentra2001

hi,My car was running fine till yesterday,but suddenly it start losing power and smoke from cathylic converter,and check engine light come out,I check the car with my OBD2 and it gives me the codeP0304,which is back fire of cylinder4,I chenge the spark plugs and erase the code,and when I tried to drive the car ,when I am pressing the gas its shaking badly and the check engine light flushed 3 times and then goes off when I am over 40km,I checke the car again with the OBD2 and its showing me the code P0304 again,any solution for that?

incorrect forum!
tune up been done?