Auto-populating text field from multiple text fields

Hello,

I am trying to find a script that will help me auto populate a text field based on the information in multiple text fields. I am working on a form that has a read-only text field that needs to be autofilled based on the information from one of four different text fields.  For example, on page 1 textfieldA is read-only and needs to be filled based on the information from text fields later in the form so that if  text2 is selected it fills textfieldA, if text 3 is selected it fill textfieldA, or text 4 is selected it fills textfieldA.  Only one field on page 9 may be selected at once and are calculated with a checkbox.  Any help will be appreciated.