Lucid Imagination has an annotated version of the “Release Highlights” for Lucene/Solr 4.0

http://searchhub.org/dev/2012/10/12/apache-solr-and-lucene-4...

Spellchecking built into the index instead of needing an ancillary one is a big feature, IMHO.

Andrzej Białecki, Robert Muir, and Grant Ingersoll will be presenting a paper on the Lucene 4 architecture: http://opensearchlab.otago.ac.nz/paper_10.pdf