A couple of very important points:
1) The default icon that the installer puts on your desktop is running blueirisadmin.exe. That crashes immediately on my system. Run
blueiris.exe instead. Works fine.
2) BI3 must be completely shut down - so if you're running it as a service, make sure it's STOPPED before installing/running BI4.
3) Both BI3 and BI4 use the same directory (c:/BlueIris/Db) for the database. This can cause errors or crashes when you try to view previously recorded clips. I had all sorts of issues until I manually deleted the db folder and restarted BI4. It took a while to rebuild, but now I can replay everything - recorded on BI3 or BI4.
4) Both BI 3 and 4 can be installed in parallel, though not run simultaneously. If uninstalling 3, do NOT allow it to delete registry/licensing data as that will bork your 4.