Re: Installing Apps on New 8.4 Tab
If you log into the same Google account with your new 8.4, then you will be able to install any of the apps you've previously purchased on Google Play (and also Amazon Appstore, if you installed that). Just swipe in from the left on Google Play, tap My Apps, and then All, and you can see all of the apps you've ever purchased or installed. Or you can just search for the app in the store.
Keep in mind, though, that some apps aren't compatible with all devices, so there might be some apps that you can't install on your new tablet.