In the encyclopedia illustration showing the Pioneer space probe paths, what is the origin and significance of the “Tenth Planet” and is it truly 4.7 billion miles away?

This is a rough approximation with reasonable accuracy. It is a Tenth Planet, meaning an undiscovered and unknown planet. This has been predicted by scientific observation of orbital shifts of those planets in the solar system behaving in ways that defy celestial mechanics applied to the known estimates of mass and the location of the orbits of the known planets. So this is a relic from the early prehistory and is yet another heavenly body that was originally orbiting the brown dwarf solar companion star that was rendered extinct, in effect, by extraterrestrial manipulation and then the Earth and other orbiting bodies were attracted by the Sun and brought into the solar system, but that is the reason this Tenth Planet is so very far out and that is because it was more weakly attracted for reasons of its low mass and greater distance compared to the Earth itself. But also that the Earth was moved deliberately by divine realm and the Tenth Planet was brought along for the ride by the gravitational pull following a near alignment of the rest of the planets in the solar system producing a kind of focused gravitational pull, to change the orbits to bend around the Sun instead of the dwarf companion.