To use a Mac Mini G4 with modern monitors you must use a DVI-A to VGA adapter.
If you only have an HDMI monitor also use a VGA to HDMI converter.
BENFEI VGA to HDMI Adapter, 1080P Converter with Audio from Computer/Laptop VGA Source to
HDMI TV/Monitor
www.amazon.ca/gp/product/B07K14NR8P/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
If you only have an HDMI monitor also use a VGA to HDMI converter.
BENFEI VGA to HDMI Adapter, 1080P Converter with Audio from Computer/Laptop VGA Source to
HDMI TV/Monitor
www.amazon.ca/gp/product/B07K14NR8P/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1