But 3.8 GB is not much and the long-term solution would be to extend the / partition from a Live CD (Trisquel's, for instance, has GParted). However, if you kept Trisquel's defaults, your /home is an XFS filesystem... and XFS filesystems cannot be shrunk. So you would need to backup the content of /home, remove its partition, extend that of / and repopulate /home from the backup.
Older kernels (images and, maybe, headers) are not automatically removed and
take quite some space. You can remove them from the package manager (search
"linux-").
