The more I think about it the more I think that Rene still is the "next generation" Sims game because they keep calling it that. So we should take their words at face value. But it's the next generation AND something different (pseudo spin-off) at the same time if that makes sense. It's both spiritually the fifth Sims game but ALSO a standalone live-service game that could just be called "The Sims" (without any numbering).
Ultimately, my prediction is that there will be an overlap period of the Sims 4 and Rene as Rene continues to build. Once a critical mass of players exist on the Rene platform, then Rene will be the go-forward live service platform of "The Sims" with Sims 4 being dropped once it's not making money anymore.