Sorry for answering in English, but my Spanish in non-existent.
You cannot download the Trisquel 4 beta yet, but you can upgrade from Trisquel 3.5 by typing the following in the terminal:
sudo update-manager -dBUT, please note that this is a beta, not a final version of Trisquel 4 and do it at your own risk (I haven't had any problems with it though).
