How can I copy text from xfce4 terminal emulator to the clipboard?

You can use clipman to sync selections and paste it anywhere using the middle click of your mouse. Just, select some text, open the terminal, and press the middle click. You can play with the options to adjust it to your preferences. To install clipman, use:

sudo apt-get install xfce4-clipman

Or, if you prefer, install the complete set of utilities for XFCE, using:

sudo apt-get install xfce4-goodies