It's very likely that the root in question does not originate with the rhododendron at all. A 4" diameter rhododendron root seems very unlikely even in a 40 year old one. Generally, the hair-like root mass becomes larger and larger with time but individual roots remain small - finger diameter or a little more is about the largest I've ever seen.

Thanks mainegrower, I will still do investigation to make sure however your response takes a load off my mind!