If you are running Windows 8 or 10, make sure you are installing with full administrator rights enabled. For Windows 8 or 10, the "full administrator rights" should be the Windows user who will be registering and using Respondus 4 after installation.
Next, make sure that your anti-virus software is up-to-date and allows "respondus4.exe" (Single-user) and "respondus4campus.exe" to run.
Finally, download the Respondus 4 software again from our website (use a different download folder) and try running the setup program again.