@Alex01121, Welcome to HP Support Community!
HP Photosmart 4780 driver for macOS 10.15 is not available from the HP website or Apple Software Update. However, Gutenprint (formerly called Gimp-Print) provides open-source basic print drivers for many printers that would otherwise not be supported in Mac OS X. To see if your printer is supported, go to Gutenprint drivers for more information.
NOTE: Gutenprint drivers are not supported by HP or Apple.
Hope this helps! Keep me posted.
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TEJ1602
I am an HP Employee