Linux

Whether or not an older machine "runs well" is highly dependent on what you're using it for. I only very recently (like, after the new year) retired a 16-year-old laptop with 2GB RAM that was running Gentoo, when I got a good deal on something that would compile gcc in a reasonable amount of time rather than needing to be left to run overnight. However, most people don't need to compile large software on a regular basis, and the old machine was still doing okay in its role as a large-screen-coarse-resolution pseudo-video-iPod, ssh client, quick lookup device for Perl manpages, emergency Internet query device, and general backup/light-use system. Worthless for gaming and somewhat sluggish on the Web, naturally, but that wasn't what I needed it for.

I'd expect anything with 4GB RAM and 4 CPU threads to produce somewhat acceptable performace on most individual webpages (multiple Javascript-heavy sites might be a challenge, though, so stick to 1-2 tabs at a time), which would make the main issue most people would have with my old laptop disappear.