Fresh install won't run

Did you try to install a second copy of codeigniter in a different folder to see if you get the same result?
Reading all this (except the dogfight) I suspect a problem with your apache installation is causing the problem.
Like Ozornick, I have installed CI4 multiple times on my WAMP environment without any issues.
Also, try a phpinfo() file in the public folder of your CI installation to see if that gives you any errors.