How long does this installation take? My iMac computer is showing the same progress bar for 4 hours?

Operating system updates and upgrades should normally take no longer than an hour to download, plus maybe another 45 minutes to install. What often happens is that a person attempts to do their download via a wireless (wi-fi) connection. This is simply too much data for wireless. What happens is that the data is either incomplete or is corrupted and won't load. I recommend that you move the download file to the trash and do another download with a wired USB connection to the modem/ router. If this is not possible, then be prepared to have to do multiple downloads until you get a clean one that works. I hope this is helpful. Good luck.