According to the release notes, some bugs concerning Lucene have been fixed:
JIT and Loop Bugs
Three bugs reported by various parties, including Apache Lucene developers, have been fixed in JDK 7 Update 1, in addition to a fourth related bug found by Oracle (7070134, 7068051, 7044738, 7077439).
Recently I saw a German blog entry that describes what you have to do to use Confluence 4 with JDK 7: http://everflux.de/confluence-4-mit-jdk-7-1901/
It's quite easy, and on my machine everything works fine tm:
Assuming you start Confluence via the shell scripts, i.e. bin/startup.sh, simply add the following to JAVA_OPTS in bin/setenv.sh:
Flush the OSGi plugin cache from $confluence-home/plugins-osgi-cache, then start Confluence, voilà. Works with Java 7