A 5-tab Google app UI seems to currently be in testing, though it's not known how widespread the test goes and how many users are able to see this new UI design within their own versions of the Google app. According to a user on Reddit who posted a screenshot of their Google app, which you can see below, Google is doing some experimenting with changing up a few things, like the shape of the cards and the search bar up top, and of course, how many tabs are at the bottom of the page, which is five, compared to the three that are currently visible in the app as of right now for most other users.
With this new design, the tabs at the bottom are the biggest change as they provide access to the different sections of the app, like the main feed, the upcoming feed which shows users anything from upcoming events to flights or packages that have been shipped and are on the way. There's also a tab for search now, which is the middle button in the screenshot below, as well as a tab for all your recent searches, and a tab for what looks like the hamburger menu icon. That last button should be significant as Google also appears to have done away with the hamburger menu in this new UI, and instead, moved it to a location in the bottom tab set.
Another thing that users will notice is that the search bar and the cards are now rounded at the corners, thus Google has adjusted the shape of them a little bit. Google has also updated the look of the icons for each of the currently existing tabs. According to the Reddit user this UI style showed up in the Google app after updating it to version 22.214.171.124, so you may need to be on that particular version for the UI to show up, remembering that just because you have that version installed it doesn't mean that you will see this UI as it is a test on Google's end, and it doesn't seem to have been rolled out to users on an official basis as part of that update.