Thank you.
Is there a way to replace A2 with something so that it is not hard-coded?
SET @TXML.modify('insert sql:variable("@NXML") after (/Attribute/A2)[1]');
No guarantees that A2 node will exist as they are optional. In this case A3 would go after A1 (or before A4). Producing:
A1
A3
A4