Hi. I have an iMAC running macOS Catalina. I have an HP4630. I have scanned a document but unable to save it to my desktop. And HP scan utility crashes.. Can you help me?

Does the Preview App Scan & save?

Safe Boot, (holding Shift key down at startup), does the problem occur in Safe Mode?

Safe mode attempts to repair Disks & clears lots of caches & loads safe Drivers, & prevents loading of 3rd party extensions, so if Safe Mode works try again in regular boot.

Maybe...try HP easy start available here;

https://support.hp.com/ca-en/drivers/selfservice/hp-officejet-4630-e-all-in-one-printer-series/5305049.

hp says your all in one printer works with catalina(10.15) and there’s a firmware update for it,too, Other than that try to reset SMC/pmu and zap pram, then as BDAQua said,give Safe mode a try

John B