That being said, while the new Hangouts 4.0 app brings a host of changes, here’s one change you might not know. According to a post on Reddit by Tutsumi (via Android Police), it has been discovered that one of the new features to Hangouts 4.0 is its ability to maintain a call even when switching networks.
As you might have experienced previously, switching from WiFi to mobile internet, or even WiFi to WiFi will sometimes cause calls or your connection to drop, but with Hangouts 4.0 it seems that your call will be maintained even if you were to suddenly lose WiFi connection or if you walked out of your house and you switched to cellular internet.
That being said as noted by Android Police, there is a few seconds of slowdown as the app hands the call over to the mobile network, but other than that it seems that the call remains active which is actually a pretty neat trick.