We know 4 is a real number but how can we prove that it is a complex number? How can be describe it in the a+ib form??
What is your definition of complex number? Does $4+0i$ fit it? Some people think of complex number as in opposition to reals, in the way irrationals do not include the rationals, but the usual definition includes the reals.
Since real numbers is a subset of complex numbers therefore we can say that four is also a complex number. The four can be written as 4+0$i$.