Pretty simple the 3rd Bobby went to sleep on the common and caught hypothermia and the regeneration kicked in and he regenerated in the fourth bobby !

You do realise that Bobby will step out of the shower and find that Lucy is alive and that he really is Peter and Bobby never existed

It was all a dream

I think you'll find that Lucys killer chopped Bobbys head off but unbeknown to them he is immortal and sprouted a new head!

Bobby regenerated

every incarnation of the beale kids only last a few years anyway!

It's like worzel gummidge

now is this his silly head?