I think there are two main options: 1) copy the 18.4 game elsewhere (rivle's suggestion) or 2) use Mod Launcher (very powerful, takes some getting used to IMO)
I went with option 1 after trying Mod Launcher only because I don't really swap mods in/out which is a big part of Mod Launcher's appeal. Here's how you do:
1) Set Steam to F: drive, it'll work fine
2) Rename F:\program files (x86)\steam\steamapps\common\7 Days To Die to old_7 Days To Die
3) Set Steam to download beta 18.4
4) Have a snack, drink a frosty beverage while it downloads/installs
5) Make new folder F:\7D2D Clients and then subfolder F:\7D2D Clients\18.4
6) Move (not copy) ...steamapps\common\7 Days To Die to F:\7D2D Clients\18.4 (so you will have a new \7 Days To Die subfolder with the whole game in it) Like this:
F:\7D2D Clients
\18.4
\7 Days To Die <-- moved here from ...\steamapps
7) Create a shortcut on your Desktop or wherever to F:\7D2D Clients\18.4\7 Days To Die\7DaysToDie.exe and use that shortcut if you want to play 18.4
8] Set Steam to "latest experimental" to download A19 (since you moved the \7 Days To Die folder in step 6, A19 will be installed completely fresh)
9) Repeat #4
Your Steam launcher will now be A19, the shortcut will run 18.4. You ought to be able to leave your save games alone, since the game will show you the version each save was created in (and will color the version # red if it's from a different version). That's what I'm doing - in either client I see both my 18.4 and my 19 worlds in the list, and they all run fine in their appropriate client. At the end of the above process both versions will be plain vanilla with zero mods installed. Re-install mods afterwards - A18 mods go in the F:\7D2D Clients\18.4 folders and A19 mods go in the normal Steam install location under \steamapps.
There are other ways to do this; the above is just roughly the steps I followed.
