If you want to try the REAPER 4 pre-releases without messing up your current v3 installation, there's this handy "Portable install" feature now:
Checking this option installs REAPER 4 to the destination folder without creating a "user folder" in %appdata% (so no files will touched/added/changed in your normal REAPER installation), all files go to the destination folder.
If you're overwriting an existing installation (REAPER 3 or 4) without having that checkbox ticked, REAPER will recognize your installation type and install accordingly.
However, keeping an eye on said "destination folder" and making sure you're installing the right REAPER version into the right folder is always recommended, but a mistake is usually not much of a problem (just install the proper version over the wrong one).
OSX:
On OSX, you can create a portable install by dragging REAPER.app from the installer DMG to its own new folder. Then create a new, empty file in that folder and name it 'reaper.ini'.
Note: for a portable install, make sure you choose a folder that you have permission to create files in! In a portable install, all of your user preferences, keyboard shortcuts, glued files in unsaved projects, etc are stored alongside the REAPER application itself. If you install REAPER portably to your normal applications folder, you may not be able to save preferences, shortcuts, temporary media, and so on.