As a guess, is it
The states of matter? (Solid, Liquid, Gas, Plasma)
One cannot be seen
Most gasses are invisible to the naked eye
One is hard to kill
Some solids are quite durable/indestructible
One is a doctor
Therapeutic liquids used in hydrotherapy act as a sort of therapy as well as ointments that alleviate pain and syrups as medicine or antibiotics can cure diseases
One is better known as a bug
A bit of a stretch but plasma wasn't known to be a state of matter so it could be considered an anomaly or bug to those of past generations
Update to include the top and bottom lines:
One real, two imagined, one real who imagined.
Solid(easily noticeable), Gas and Plasma(unseen and misunderstood in the past), Liquid(not quite sure) respectively
One performer, two fighters, one sidekick.
Plasma(usually present as bright, shiny things that attracts people's attention), Solid and Liquid(fighting with Plasma for the spotlight), Gas (the most unnoticeable of the 4)
One living, two ageless, one dead but immortal.
One living: Solids are broken eventually by time as with other living things.
Two Ageless: Gasses and Plasma always look the same even overtime.
One dead but immortal: Liquids go through the water cycle and are "rebirthed" over and over again.