You need to call the function f, like this:
g=@(x)x*f(x)+4
f is a function handle, not a numeric value. Function handles cannot be multiplied. Only when you call the function handle with its input does it return a numeric value, which can then be multiplied.
This is just like any other function in MATLAB:
5*sin % error, because sin is a function not a value
5*sin(pi) % okay, the function sin returns a numeric value
