Both pneumonic and bubonic plague are caused by pasteurella pestis, now Yersinia. There arent very many black rats around rattus rattus, now, which is why it is rare.
Pneumonic usually occurs at the end of an epidemic, when the PPestis has been rendered more pathogenic on account of passage through humans, and is direct through sputum, and the lungs and inhalation.
WHy did it kill only 4 ? Why does any epidemic end ?
Oh, and by the way, when you talk of the last cases, I am sure you mean in the UK. Plague was described in Vietnam in the sixties. - about 100 deaths.
In modern terms, it looks as though plague is just about as infectious as anthrax, In therecent outbreak in the US, I dont think there were any humna-human contact cases.
I hope all the would be terrorists are reading this (and thinking of giving up)