This method is most likely no longer suitable for a plone WSGI setup. With stable releases of Zope 2.13 coming out and Plone 4.1 almost ready for alpha, it'd be best to start there and ignore this post.
I'm not going to go into detail of the wheres and hows everything is done. This post expects you to know a bit about plone, zope and buildout. Maybe I'll be more detailed later. Use this if you want to save yourself a lot of time in getting a working setup with Plone 4 and Zope 2.13. Zope 2.13 adds native WSGI support in Zope. I tested it a bit and seems to work well but results may vary and I'm sure there will be a more supported way to do this soon.
Make sure your buildout extends the http://download.zope.org/Zope2/index/2.13.0a1/versions.cfg versions file.
You'll need to checkout Products.CMFCore, Products.PluggableAuthService, Products.TinyMCE and plone.locking from svn. You can use these locations right now until there is a new release,
You'll also need to add these packages to your develop buildout section.
Extra Versions Pins
You'll need to pin these versions since Zope 2.13 doesn't pin version that plone's setup used to assume were pinned,
Add these extra version pins
You'll also need to add extra add dependencies to your buildout that the Plone 4b4 egg doesn't require and should.
You can take a look at my previous post for doing WSGI Zope2 for guidelines on how to setup the paste config and such.