Project Rene is The Sims 5. They just won't call the game that because the business model is changing, and they'll abandon the numbering.
They have already referred to Project Rene as the future of The Sims, the next generation. So it's pretty evident that, in some ways, it's The Sims 5.
The Sims 4 became the best-selling game in the franchise and with the largest player base in history, either because the base game is free, but also because the game is available on consoles. It's a very large audience.
It wouldn't be smart for them to abandon The Sims 4 considering its audience, and also the fact that Project Rene will start without all the content we currently have in The Sims 4. There are 9 years of content, and an audience that spans PC and Consoles.
The audience and content created is much larger than in previous generations, making it not worth abandoning a game with so much content released to start all over again in a new game.
Catharina Mallet, General Manager at Maxis, said that its games can last up to 15 years. I take this as a clear insinuation that The Sims 4 could be this long.
Project Rene will probably be released between 2025 or 2026, by which time The Sims 4 will be around 11/12 years old.
It is very likely that when Project Rene is released, they will continue releasing content for The Sims 4 for at least three years.
Three years of simultaneous releases is enough to make Project Rene a strong and attractive game, to show what it came from, and also the strength of its content, and then, yes, to finish The Sims 4.