Looks like having only 4 Gig space would make it tight. :\ I have manually removed all contents of the trashdir with no ill affects site wide, but, then again, dunno if a teacher attempted to restore a file deleted or not ... none have reported an issue.
But I do have a question ... and this might free up some more space as well ... what are the 'numbered' directories you show in the screen shot? Those look like the old =<1.9 course ID directories. The /1 was the front page of =<1.9's. The /1 can removed for sure as you can't use it to replace front page anyway. But do look in there to see what's there before removing.
With things that tight space wise ... every little bit helps! ;)
'spirit of sharing', Ken