Google's Messenger application is their answer to the wealth of options available in the Play Store for third-party SMS apps, and it offers up quite a few nifty features that users have come to fall in love with, not the least of which is the customization capabilities. Of course, the app has been made all the better with Android N Developer Preview versions as Google has added in Android N specific features, such as direct reply, and it seems that the fourth developer preview for Android N improves upon what was already a popular functionality with users, which is likely going to get more people to use Messenger or at least give it a try.
As for what the fourth developer preview of Android N brings along, it's now possible to expand the Messenger notification and get more than just an option to quickly reply to that incoming message. The notification, if pulled down, will now also show you a brief few messages from the conversations along with time stamps, instead of just the message that came in most recently. If you're planning on using the direct reply functionality quite often and as a means for having a conversation without having to actually open up the Messenger app, then this will prove to be a very useful change for you.
Also as part of the latest version of the app which comes along with the fourth developer preview for Android N, if you receive a notification from Messenger and then reply to it from the notification, the app notifications will longer be dismissed and will stay in the drop down panel until you dismiss it yourself. What's more, your reply will then show up in the notification when extended to show the conversation view. As long as you're running any version of the Android N developer preview, you should be able to access these changes to Messenger if you have the newest version of the app. Since Google has only sent this version of Messenger out with the most recent version of the preview, you can grab the APK from below if you want to be updated without having to change your preview version.