You can easily continue works started in Krita 4 in Krita 5 and must not be afraid if you take precautions. Therefore, I ask you to make sure to back up your settings files and your resource folder before installing Krita 5 beta versions, it probably would be best to read @tiar’s hints on backing up this data >> Before you start (how to back up things).
A backup is a good way to don’t let something unexpectedly go wrong and turn your test into a negative experience, that would be a pity.
For testing, I recommend using the portable versions (the ZIP versions). This is the direct download link to the current Beta 5.
If you want to separate Krita 5 absolutely clean from your Krita 4 installation, you can do this by using another Windows user account. I have this set up on my end so that I can use different Krita versions at the same time, this works through the capabilities of Windows’ “RunAs” command. A description of how to set this up can be found here, it is a bit technical, but it should be understandable. If you have any questions, I will try to answer them.
The, in my eyes, only disadvantage is that the versions started this way do not allow drag and drop access and files you double-click on will still be opened in Krita 4, but you can of course open the files with the dialogs in Krita itself. But that’s just what a test installation is for, to keep things separate.
On a PC with 32 GB RAM you should be able to use 2 (good) to 3 (maximum) versions at the same time (but after that it gets “tight” I think). But you can only paint in one Krita at a time anyway, but such a setup is useful for testing different versions.
Michelist
Addition: The limits of this method are determined by the available resources. At the end of the description linked above you see in a screenshot 7 versions at the same time on one desktop, it is not a composite photograph.