Is it a larva of some kind? With jaws like that I'm glad it's only 4mm long!
Yes, it's a lacewing larva. They decorate their backs with the husks of their victims as a form of camouflage.
That's quite gruesome! Is it possible to identify the species? (I know even adult lacewings can be difficult.)
The larvae are often well marked so I'm sure it would be possible to make some progress though I don't know how hard it would be to get to species. I don't have any references for the larvae but there is a UK recording scheme where you might be able to find out more: http://lacewings.myspecies.info/