The biggest change in OS X 10.10.4 is the elimination of the discoveryd process that's been accused of being responsible for some significant networking and WiFi bugs within Yosemite. Discoveryd is a networking process that was introduced in Yosemite to replace mDNSresponder, but it caused problems like slow wake from sleep, failures to resolve DNS names, duplicate machine names, WiFi disconnects, excessive CPU usage, battery drain, and more. In 10.10.4, discoveryd has been replaced with the older mDNSresponder process.
http://www.macrumors.com/2015/06/30/apple-...s-os-x-10-10-4/