I'm having this same problem. I've tried 3 or 4 different paths for upgrading, followed this guide with a local version of my site in WAMP and still have the same issue. Re-downloaded the latest IPB4 version. At this point the only thing that immediately comes to mind is that on my 3.3 install I had IP.Content, and I didn't renew IP.Content for 4, so the upgrade doesn't include that.
EDIT SOLVED: I've had this problem for 3 or 4 days. Literally a minute after submitting this, I go to IPS Support, the little tooltip says to try the support tool in the AdminCP. I try that, it says there might be corrupt cache data or something, and it clears that. And... my test site is working.