Amigaz are u sure you have read the manual?
I did not even manage to install OS4 before reading it (first time in 15 years I needed to read a f"n manual before managing a software installation).
When you install it be sure to select the graphics board you have, it is on AGA by default.
I did get errors on 3 installations of 5. In final part OS4 installation copies a couple of files to the harddrive, I got an error there that the installer could not copy any files so I had to format/reinstall because I did not know which files where needed, slightly annoying but at least now it is installed on a quiet HD.