One can rightfully argue that Samsung's Android Pie rollout is late compared to the competition. However, the company has been doing an excellent job of trying to bridge the gap and a few years down the line; we can expect to see Samsung update their devices alongside companies such as OnePlus. The ongoing Android Pie beta program for the Galaxy S9 and S9+ is presumably in its last phase as its fourth iteration is rolling out today to some users in Germany and the UK.
This is good news, as the original Android Pie beta was in a state of near completion after four iterations. We can expect a full-scale rollout by January next year, as promised. The primary purpose of incremental updates to a beta build is to quash bugs introduced by a previous release. Since it is a beta, the initial builds are a broken mess which gets better over time. Samsung is engaged in a never-ending game of whack-a-mole with their software and some issues persist even through the final release.
In the previous iteration of the OneUI-based Android Pie beta, some apps linked to your Samsung account wouldn't work. Additionally, message notifications wouldn't go away from the notification shade even after opening the message in question. Similarly, the phone app crashed when you made a call and making a phone call caused the screen to flicker. All the aforementioned bugs have been quashed, and hopefully, the next iteration doesn't break anything.
Naturally, the update is available only for users who participated in the beta program. It would be overly optimistic to assume that this update wouldn't introduce new bugs of its own as they almost always do. The update is a 324MB download that will be available over the air in the next few days. Additionally, you can attempt to trigger it by navigating to the 'Download and Install button' in the Settings > Software update menu on the device.
It is unlikely that you'll be able to participate in the beta this late, but if you want to try your luck, head over to the Samsung Members app (Samsung+ for users in the US), navigate to notices, tap on the OneUI Beta, and you'll receive an OTA update that'll bump you up to Android Pie.
News Source: sammobile