The fourth maintenance release includes approximately 50 bugfixes.
According to the announcement, the majority of the bugs were contributed during Zend's Bug Hunt Days two weeks ago.
The three fixes highlighted in the announcement are:
1. "An important serialization improvement in Zend_Amf that has been benchmarked as providing 200-300% faster serialization of large datasets."
2. "A fix to Zend_Loader::isReadable() to fix a regression found on Windows platforms. This fix should eliminate most if not all raised warnings that occurred during resource and plugin loading."
3. "A patch was applied to Zend_XmlRpc_Value to make value generation more efficient. Benchmarks and profiling show enormous changes on large datasets: in one example, memory usage drops from >1GB to 20MB, while dropping from execution times of >60s to around 10s."