The Sims "Project Rene" was going to be a sort of multi-game experience based on a multiplayer style game, a classic singleplayer sims, and other gamemodes that would coexist with the classic Sims 4 game. That plan is dead. Instead, TS: PR has been cut into pieces, and the singleplayer mode is going to be sold as a straight up TS4 successor which is named Project X internally.
Its main features are Open Neighborhoods (not a complete open city like TS3, but only a sector of it) with busier streets, detailed graphics akin to the early showcases, and SOME TS4 aspects being carried over to TS: PX. Because the Rene base wasn't built to be a singleplayer experience at its core, it's much simpler than TS4 currently, but it wasn't made as "The Sims 5" even if it's branded as such in the end.
So, why kill TS4 if EA meant to keep it alive forever? Simply, the engine is too old, too messy, and they don't want to support it anymore because the upkeep costs are too big for what they can do with it. And as many people suspected, the Q/A and development of new TS4 content is becoming much harder with the engine being a mishmash of code from the early 2010s.
Also, they want to identify mod-related issues easier because people tend to report them as "game issues".
EA also intends to add a "fan-favorite feature" to the base game, with some Sims 4 planned content moving over to the new game.
It's said to be coming out at the end of next year.