diff --git a/README.md b/README.md
index 46a929d..2d96147 100644
--- a/README.md
+++ b/README.md
@@ -80,6 +80,9 @@ link followed by "Download Zip."
 
 Similar steps are possible under linux and MacOS X.
 
+# Breaking changes with 4.x
+Support for non-cryptographic hashes has been added in 4.0.0. For 128 bit hashes, md5 has been replaced with xxh3_128, one of the [fastest](https://github.com/Cyan4973/xxHash) non-cryptographic hash functions. Details of the benchmark runs can be found [here](https://github.com/joseph-fox/python-bloomfilter/pull/38). Files generated with earlier versions of the module *will not work* with this version. Consider re-generating them using the latest version optimized for speed.  
+
 # Installation verification
 Type `pip show pybloom-live` from a command prompt. Version should be 
 2.2.0 as of 2016-12-11.