This is a bit old, but I believe you can do the following:
notes = Notification.objects.filter(user=self.user)[:4]
Notification.objects.exclude(pk__in=list(notes)).delete() # list() forces a database hit.
It costs two hits, but avoids using the for loop with transactions middleware.
The reason for using list(notes) is that Django creates a single query without it and, in Mysql 5.1, this raises the error
(1235, "This version of MySQL doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery'")
By using list(notes), we force a query of notes, avoiding this.
This can be further optimized to:
notes = Notification.objects.filter(user=self.user)[:4].values_list("id", flat=True) # only retrieve ids.
Notification.objects.exclude(pk__in=list(notes)).delete()