I've done several of these installs and they can take a really long time. First you have to uninstall the previous version and then reinstall this version (required due to its dependency on Java 7). The install can take forever, particularly under certain circumstances like the previous install had a large log file.
If you want this to go a lot faster, after uninstalling the previous version (don't forget to export your schedules and groups), copy whatever you need from the FileMaker's Library (e.g., log file, databases (including backups), extensions, etc.) to your desktop or documents folder. Then erase the entire FileMaker Server folder in the Library (note that the uninstall does not remove these folders). The 12.0v4 install will go much faster then having the install go through the previous FileMaker Server library folders and updating things. I don't know why, but FMS 12.0v4 installer can spend an hour or two if the old folders are still there. I'm sure there is something logical it is doing, but it does not benefit me and keeps the databases down for a long time.
Just my 2 cents after doing several of these installs.