E-Signature Formula with Nested IF

I am trying to use a formula field to generate a date, with a nested IF statement. 

Using a dropdown field whose options are 1,2,3,4:  IF 1, date signed + 36 months, IF 2, date signed + 36  months, IF 3, date signed + 24 months, IF 4, date signed + 12 months.  It can also be years.  I cannot get the prompt generator to complete a formula, other than a simple date signed + x months. 

Please help