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.

The TPM chip is the issue here, and not a requirement under Linux.

Linux runs just fine in 4. Or much less. It depends a lot on what you use it for. My 486 had a whooping 32 Megs of memory and ran Linux just fine.

Regarding MS, the main problem is the changing of the goalpost. And I'm not so sure there's even any point to the whole TPM thing anyway.