paul@32
That is an amazing coincidence. This supplementary challenge, if too obscure to be fair, has been worth it just for your AARGH.
The challenge is still on to find the other answer (+ a dubious one). To recap: TOAST, CLIFF, RADON, AARGH.
Five-letter words only, what else could have been chosen instead of AARGH to echo a connection in that quartet. I would expect some simple googling to find the answers.
Hint: the key is in the last three letters of each word. They are the last three letters of a unique quartet.