You can do this with some simple FormCalc script by selecting button1 and editing it's Click event in the Script Editor palette. Here's the script you could use:
FSN2 = FSN1
PartNo2 = PartNo1
ItemD2 = ItemD1
Serial2 = Serial1
I'm wondering how your form is setup, however, because this seems like a case where you would want to be using dynamic subforms where each item would be a subform (each containing the four text fields described in your post). You could then setup your forms such that on the click of a button, a new item is added to the form (which could easily be pre-populated with the information entered in the last item). If this sounds like it applies to your form, I'll be happy to go into more detail.
Stefan
Adobe Systems