Microsoft: "We can't just call it the XBox 4, because it's competing against the PlayStation 5 - people are stupid and will think it's less powerful or older..."
Also Microsoft: "Yeah, this name is confusing, useless and possibly worse than Wii-U, but people are smart, they'll figure it out..."
I'm mainly interested in backwards compatibility. So far I've seen a quote from Microsoft saying that the next XBox (series X) will play everything the XBONE plays. So all XBONE games, many X360 games, and a few original XBOX games.
Sony, I don't think has officially said anything yet. It's assumed the PS5 will play most or all PS4 games but that's about it... PS3 games seem unlikely. Via streaming or digital (buy-it-again) downloads, probably yes, but not just putting the disc in the system and playing.