Thanks god, there’s a buildout recipe (we should create a Linux distro based on buildout someday, alecm ;) to build Python from source in an isolated environment. Steps:

1. svn co https://svn.plone.org/svn/collective/buildout/python/ buildout.python
2. cd buildout.python; python bootstrap.py
3. Edit buildout.cfg to fit your needs. You might want to comment out the references to the Python versions you don’t want to install.
4. bin/buildout
5. cd to-another-directory; paster create -t plone3_buildout plone4.
6. Enter “4.0a1″ (without quotes) when asked about “Which Plone version to install”. Make sure you have the last version of ZopeSkel (2.14.1 while I’m writing this – easy_install -U ZopeSkel) or the generated buildout.cfg won’t be valid for Plone 4 otherwise. Thanks to MatthewWilkes for the pointer and of course for the ZopeSkel release. :)
7. path-to-buildout.python/python-2.6/bin/python bootstrap.py
8. bin/buildout. If you get tons of lines in your console output about fetching distribute, see this post by Reinout van Rees.
9. You’ll need to install PIL to run Plone, so do either:
   1. python-2.6/bin/easy_install-2.6 http://dist.repoze.org/PIL-1.1.6.tar.gz, or
   2. Add PIL or PILwoTk to any of the eggs sections of your Plone 4 buildout and re-run bin/buildout.

If you stumble upon an error message similar to:

Downloading http://dist.repoze.org/PIL-1.1.6.tar.gz
Processing PIL-1.1.6.tar.gz
Running PIL-1.1.6/setup.py -q bdist_egg --dist-dir /tmp/easy_install-Fev48C/PIL-1.1.6/egg-dist-tmp-SNtCRu
In file included from decode.c:608:
libImaging/Zip.h:11:18: error: zlib.h: No such file or directory
In file included from decode.c:608:
libImaging/Zip.h:37: error: expected specifier-qualifier-list before ‘z_stream’
error: Setup script exited with error: command 'gcc' failed with exit status 1

try “aptitude install zlib1g-dev”. Thanks davisagli!

The previous procedure should work… But it didn’t in my system. :(  I get an error in the buildout.python bin/buildout:

SystemError: ('Failed', 'patch -p0 < /somepath/buildout.python/parts/readline-patch/readline.patch')

I’ve already tweeted fschulze (who is presumably the author of the collective.buildout.python stuff, according to comments in the #plone IRC channel) about this issue. I hope it will get solved soon! :)

In the meanwhile, removing or commenting out the stuff about readline.patch in the collective.buildout.python buildout.cfg before running bin/buildout appears to be a valid workaround, at least for me.