Autofill a spreadsheet

I have a spreadsheet I send out as a letter and whos reference number ascends chronologically. This reference number is shown on the form in a cell, say E4. I want to be able to type in the reference number on a blank template of the letter and be able to see the letter I desire.

Example, I have spreadsheets 1, 2, 3, and blank form letter. I want to type in 2 in cell E4 of the blank form letter and then see letter#2. Is this possible.