Normally with Google services, the Android platform is the first to see any new updates. That hasn't been the case with Hangouts version 4.0 which has already been sent out to iOS users while those on Android are still waiting for the update to roll in. Whether or not you use this as your main messaging app there are some features coming along with it which are sure to delight even the most modest Hangouts users, like Android Wear support with a proper app, a redesigned app UI with more material design as well as the usual performance and bug fixes.
Back in the beginning of June the first reports of Hangouts getting full-on Android Wear support hit the public and since then many users have been waiting for Google to finish the changes and get things rolling. While the presence of a proper Android Wear app is a certainty at this point, things have now come down to a waiting game for eager users who wish to send messages from their wrists. And, you will be able to send messages from your wrists but not like the current status where you can reply to a hangouts message that comes in. You'll actually be able to initiate a Hangouts chat message by simply speaking the "OK Google" command and then telling your watch to send a message to one of your contacts, after which you'll be give the option of replying with your voice by speaking your message or drawing an emoji on screen before letting it send.
It's this particular detail, the Android Wear app's voice messages, which is said to be holding back the release of Hangouts 4.0 to Android users. The app is still reportedly being tested internally so things aren't quite ready for the public, but continuing to test the app to make sure things work as smoothly as possible before release makes a strong argument for Google's sake as to why things are taking a little bit longer for their own home grown OS then expected. Nevertheless, things are likely not too far off now so if you've got an Android Wear device and you use Hangouts, you'll soon be able to take full advantage of it without having to touch your phone.