As per the author's statement, using ZopeProfiler together with Plone4 is unsupported. It really is. First, get a current version of ZopeProfiler instead. Implement in your buildout as usual and run buildout. In the relevant instance's (eg. secondary) zope.conf, one has to enable it, too:

enable-product-installation on

You also need to fix the output from the pstats module. In Debian, this is located at /usr/lib/python2.6/pstats.py. Copy to your virtualenv's lib/python2.6 and manually apply the patch mentioned here: http://bugs.python.org/issue7372

After that, following the instructions generally works, except for that the site now runs orders of magnitudes slower, and (at least) I get this error when trying to view the stats (sample traceback):

2011-05-04 13:47:56 ERROR Zope.SiteErrorLog 1304509676.940.218731970327 http://localhost:9082/Control_Panel/ZopeProfiler/showHigh
Traceback (innermost last):
  Module ZPublisher.Publish, line 127, in publish
  Module ZPublisher.mapply, line 77, in mapply
  Module ZPublisher.Publish, line 47, in call_object
  Module Shared.DC.Scripts.Bindings, line 324, in __call__
  Module Shared.DC.Scripts.Bindings, line 361, in _bindAndExec
  Module App.special_dtml, line 185, in _exec
  Module DocumentTemplate.DT_Let, line 76, in render
  Module DocumentTemplate.DT_Util, line 202, in eval
   - __traceback_info__: stdnameRe
  Module <string>, line 1, in <module>
  Module Products.ZopeProfiler.ZopeProfiler, line 237, in getStatistics
  Module pstats, line 353, in print_stats
ValueError: I/O operation on closed file

I've seen the latter error on various other occasions as well, esp. when a long time has passed between the original activity and the display of results (eg. when running ExternalMethods). If someone has a fix for that, I'd highly appreciate it!