The advice about the Combo was wrong you can only install any Catalina Combo update if you have already installed a
previous version of Catalina.
I take it you downloaded Catalina recently, therefore the version of Catalina that would be installed would give you the latest version which is 10.15.4. If you were running an older version of Catalina and install the Combo it would take you to 10.15.4.
So no need for the Combo at all.
Can you boot to your Recovery HD, restart your mac and press and hold down the Command and R keys, the mac will boot to your Recovery HD, you should see a Utilities panel and click on Disk Utility, press continue and run Disk Utility> First Aid on your Macintosh HD. If you get any errors run again until you get the green tick.
Restart normally to boot back in 'normal' mode and try the install again.
If you can connect your mac to your router via cable rather than rely on Wifi, just in case the mac is calling home.
If there are still problems try installing Catalina in Safe Mode.
How to use safe mode on your Mac – Apple Support
