Although it isn't Hangouts 4.0 for Android, Google's instant messaging service on the web (and Chrome extension too) has been updated with a much easier way to share images in conversations, via good ol' drag and drop.
Prior to today, a few extra clicks/a bit of faffing about were required to attach an image. This new approach shaves seconds off what should be a simple task anyway, and is much more user friendly.
It's pretty slick, but our eyes are still set on that impending, hopefully satisfying Android update we've been waiting oh so long for. We'll keep you posted when that develops...