When I turn my iMac on it says 4 viruses have been found

Mac's have built-in security and do not get PC viruses.

macOS - Security - Apple

The problem is, you have installed some type of third-party junk-ware that is now causing problems.

If you know what the bogus third-party junk-ware is? Uninstall it as per the developers instructions.

Then run EtreCheckPro on your iMac Pro.

https://etrecheck.com/en/index.html

And post back here with your iMac pro's EtreCheckPro report.

How to use the Add Text Feature When Post… - Apple Community

Download and run the free version of Malwarebytes. It was developed by a long time contributor to these forums and a highly respected member of the computer security community. It's designed to locate and remove malware and adware.

The free version is sufficient for most of us.

+1 Den.Thed is correct!