Setting the export Value of a drop down menu to the value of another field

I forgot to add this thread:

https://community.adobe.com/t5/Acrobat/Show-Default-Value-in-Text-Field-Until-User-Inputs-Data/td-p/10003538

It seems to achieve better what you are trying to do

If I understood correctly,

You can assign an export value to each listed item in the dropdown. It is static but this simple method that I've tried before works.

Unless you need Text1, text2 and text3 to be visible to the user it will be unnecessary if you only use  dropdown menus instead of text fields with a predefined value.

So, in the dropdown  field that you want to display the export value, rename it to "exported value" (for example); In that dropdown field you are going to tick the option that says "allow user to enter text" (or similar option); then add the following custom calculation script:

event.value = this.getField("Item1").value+10;

And then in Text4 field add a custom calculatiom script like:

event.value = this.getField("exported value").valueAsString;