At the moment the nightly build installer for Windows is a full package. The safest choice is to uninstall v4 first, but it’s possible to run both simultaneously. That would require some environment variable tweaking, though. See Any ultimate Guide on how to use KiCad 4 and 5 on one System? and https://docs.google.com/document/d/1Rq8i2Ay7qpGpffaj-AQmE-Xp88ikHhgyt0Ygpi8717o/edit?usp=sharing.
Also you have to be aware that you have to migrate projects made with v4 to v5 and then you can’t migrate them back. I you are just playing with projects for practice you can do the migration or start new projects. If you have something which you want to get finalized you could do it with v4 or migrate a copy and try to go on with v5.