Power Mac G4 Model no. 8570

Note that the USB ports on those old Macs are USB 1.x ports that top out at a speed of 0.012 Gbps, and were never designed to carry video. (By contrast, USB-C (DisplayPort Alt Mode) supports video data transfers at 16.2 Gbps.)

Between the excruciatingly slow speed of USB 1.x, the fact that you need to install special software to use modern "workarounds" like DisplayLink, the odds against being able to easily find "workaround" software compatible with such old Macs, and your lack of a monitor to use in installing and configuring any "workaround", I don't believe you are going to be driving any monitors over USB.

According to MacTracker, Model Number M8570 matches

• Power Mac G4 (Mirrored Drive Doors) – 8/2002 to 1/2003
• Power Mac G4 (FW 800) – 1/2003 to 6/2003
• Power Mac G4 (Mirrored Drive Doors 2003) – 6/2003 to 6/2004

None of those have HDMI video output, but according to MacTracker, all have one DVI output and one ADC output.

ADC is a "proprietary modification of the DVI connector" and adapters to go from ADC to standard video interfaces may be hard to find (nobody has made any in a long time).

DVI to HDMI adapters should be easy to get, even if you need to order one from Amazon [Marketplace] or another Web site.