Unless you mean adding a new MIP level with higher resolution
That is indeed what I meant. You don't need a 4K screen to get benefit from having 4K textures over 2K textures. When was the last time you saw a game in which texel size never exceeded pixel size on a 1080p screen? Not even Rage with its megatextures can manage to do that. Sure, things look good at a distance or at 60mph, but if you get close up and inspect things you'll see the large texels and compression artifacts.
Err, the low res textures and artifacts are due entirely to disc size limits. There aren't any particular technical limitations to virtualized textures, you can have a one texel to one screen pixel texture size (based on near clipping plane) if you want. There's also no real problem with creation of virtualized textures, in fact it's better all around than the "standard" way of doing things, as there's no memory limits on size or variety of textures. Nor are they required to have all pixels stored uniquely, you can repeat textures all you want, in fact Trials Evolution already does so, and is a fairly small downloadable game. You can also eliminate unwrapping UV maps for your artists if you want to do so (GDC... 11 or 12, a Lionhead paper).
It is pretty much the closest thing you'll find to a silver bullet in terms of texture memory problems. Even with 8 gigs of ram there's still a lot of benefit to using such a system. But as I said, I can already just imagine other ways of filling up a ton of ram quickly for a game.