I thought I would create a specific set of instructions since so many would like to check out the V4 prerelease without mucking up their V3 install. Its much harder to explain than it is to do. It really isn't that difficult at all if you follow a few basic principles.
[IMPORTANT]
I've done limited testing with this on XP and Win7. If you feed yourself with Reaper, wait until someone who doesn't confirms the instructions below so I can edit anything I missed. That is entirely possible since everyone's configurations can vary wildly; it's also late and I'm tired.
Use this as a guide, not the definitive answer. I'd like to see someone confirm this and give their blessing first to be safe. Outside of that, I think it will work fine.
[/IMPORTANT]
1. Backup your existing V3 install and settings
This is a safety step, don't skip it. Copy the following folders in their entirety to a safe location.
==> C:\Program Files\Reaper
==> \AppData\Reaper (Start > Run > %AppData%\Reaper)
or C:\Users\UserName\AppData\Roaming\Reaper (Vista/Win7)
or C:\Documents & Settings\UserName\Application Data\Reaper (XP)
==> My Documents\Reaper Media (probably optional, should only contain WAV files & peaks)
2. Install Reaper 4 Pre Release
==> You must check the "Portable install" checkbox!
==> You must change the installation path to something other than the existing V3 location!
I recommend a USB pen drive or some other folder you have full rights to without any UAC protection. A folder on your desktop named ReaperV4 will even work for this. I suggest not installing it in C:\Program Files\ReaperV4 to prevent any chance of Windows of secretly redirecting any files to V3 locations just to be on the safe side.
[Default Installer Settings]
[V4 Coexistence Installer Changes]
==> Click next etc, and allow the install to complete.
At this point both versions are pretty much completely Side by Side and share zero common settings. If thats what you want, your done, Run V4 alpha and choose your sound card, VST folders etc. Stopping here means your pretty much testing Reaper 4 prerelease as-is out of the box with no customizations and zero V3 settings. If you want to go further and have V3 and V4 working while having V4 using the V3 settings, themes etc. continue with step 3 below..
3. Go ahead and launch V4 if you haven't already
Import your license if you wish. Go ahead and choose the soundcard and test basic functionality such as hitting play etc. I suggest spending 15-20 minutes at this stage checking out new features etc. If you import a bad setting over from V3 you wouldn't want to think it was a V4 bug etc. Be methodical, save some grief.
4. Copy V3 settings into the V4 folder
==> If your a cowboy copy them all at once, if more cautious copy them category by category test, shutdown reaper, copy the next, launch test and so on.
==> Copy the following folders into the V4 folder you installed to. If made the V3 backup copies in step one, just use AppData folder as the source:
ColorThemes
Cursors
Data
Effects
KeyMaps
TrackTemplates
ProjectTemplates
FXChains
Reaper.ini (optional) <== I don't really think this is needed. Make a backup of the V4 copy first.
5. Point your VST folders if needed.
If V4 isn't showing your 3rd party plugs, open Options > Preferences > VST > Add and add each folder that V3 has. In fact before you do this. Run V3, go to the same preference and copy the plugin paths to the clipboard. Close, V3, Open V4, Return to Options > Preferences > VST > Add and paste 'em in. Click Re-scan and let it scan all the Dlls. Note: I
think Reaper is a single instance application. This means if V4 is open and you click on the V3 shortcut, V4 may pop into focus while V3 silently exits. (same vice versa). So don't let this throw you and assume V3 is busted. Shut down one version before attempting to launch the other one.
SUMMARY
By this point you should have a V3 install that is untouched. It should function as it always has because nothing about it has been changed provided you followed all the steps above. You should also have a functioning V4 prerelease version containing 99% of all settings that are available in V3. When you are done using V4 and need to remove it, simply delete that single ReaperV4 folder. If you want to upgrade to the latest alpha release just download and be sure to install the update to that same V4 location (overwrite don't delete) and all will be good. If you open that V4 folder, right-click on reaper.exe > send to > Desktop (create shortcut) and you'll have a nice V4 shortcut on your desktop.
Remember, you backed up all the V3 settings right?
So, you can always copy these back over to their original locations if you phat fingered the install and installed over V3. That should get you back to your original V3 confing in an emergency. If you open an important project, you can make a copy of that project if you want to be really cautious or save any changes to a new V4 project. Of course you have all your important stuff backed up anyway.
Karbo