The story of Matilde and Bruin has all the makings of a classic fairy tale: the villain Cormoraunt is conquered by Bruin, Bruin and Matilde have a happy but childless marriage, and a child is prophesied to come to them, but from an unknown source.
That source turns out to be Calepine, who runs across Matilde after he's just saved an infant from a wild bear. And voila: infant needs a mom, mom needs a baby. And even though the child doesn't biologically come from Bruin, the poem hints that he still has a connection since his name, Bruin, means "bear" in Old English. So, symbolically at least, Bruin is a part of his child's origin.
The story of Bruin and Matilda may be out of a fairy tale, but the importance placed upon having an heir not just for personal reasons but for political ones too reminds us again in Book 4 of the constant intersection between what's personal and what's public.