I've just merged Eoghan's work on the Solr 4.2.1 upgrade into master... so VuFind 2.0 will be released with Solr 4 bundled in.
I've deleted the solr4x branch since we no longer need it. We should create new temporary branches for future upgrades.
Let me know if you have questions/problems. You'll probably want to reindex your data next time you pull the latest code from Git.