I wondered if somebody would mention Nissan as diesel engines running out of control has been a persistant problem on the original X Trail and led to 'Honest John' changing his recommendation from a 'Buy' to 'Avoid' as Nissan failed to address it, or help people out sufficiently.
Its rare in an Alfa, and the diesel engines have, somewhat ironically, restored Alfas reputation with engines after the poor record of the TS /JTS (Oil and cambelts) and the mixed record of the V6 (cambelts) but the very nature of a compression ignition engine means that once its going, it won't stop until it has consumed all the fuel from any source it can get it, hence they will consume sump oil until the engine destructs. If a diesel valve sticks open, then this clearly has a similar effect.
Of course I hope that Alfa will help out, but as to a legal liability I'd be very doubtful at 4 years. Every body is very short of money though, so handouts may be thin on the ground.
Good luck with your claim