Although I know how to add Page No.

But the problem is if I have 20 pages in my documents. I want to add page number from 5th page.
But When I went to page no 5 (Make its my page 1 & No page numbers from 1-4) and then add page number. it makes 5th page and on pervious four are still numbered.

Although I know how to add Page No.

But the problem is if I have 20 pages in my documents. I want to add page number from 5th page.
But When I went to page no 5 (Make its my page 1 & No page numbers from 1-4) and then add page number. it makes 5th page and on pervious four are still numbered.
Put in a section break after page 4 (Insert, Break), then unlink the headers and footers in Section 2 (so they do not say "Same as previous"). Then add in your page numbering from Page 5 - you may need to reset the number to start from 1.

A very good pointer, Sandre!