I also had difficulty installing ver 4.0.4. In brief (as you already have read) you have to first uninstall ver 3.x.x manually.
I found this post http://forums.eeye.com/forums/p/591/2506.aspx#2506 by Inicula that worked for me. (Since I don't know how to insert a working link to this post, you can find it under Troubleshooting, Cannot Uninstall Blink Personal..., 08-11-2008. lnicula, 4th post.)
Referring to Inicula's 7 steps, I also ran CCleaner before and after Steps 3 and 7. It got rid of a few dozen additional eEye registry keys.
If I remember correctly, I booted in safe mode at Step 4 and deleted the C:\Program Files\eEye Digital Security\Blink folder directly without having to download and run Chown. This allowed me to skip Steps 5 and 6. I was then able to instal ver 4.0.4.