Why do para subsituted benzene show 4H double doublet on H NMR?

Answer 1

In order to say that it is para-substituted you need to have 2 substituents.
If the two substituents are the same, then you should get 1 singlet peak (the symmetry would render all H equivalent).
If the two substituents are different there is still some symmetry: there is a plane of symmetry perpendicular to the plane of the ring, on which the para-substituents are placed.
Thus the 4 H are grouped into 2 types of non-equivalent H.
Each H will have 1 non-equivalent H close to it, so the splitting is 1+1= 2. Thus each H would give a doublet. Since you have 2 types of H, you get 2 doublets with each doublet corresponding to 2H (or if you prefer, a double doublet corresponding to 4H).