How can I set system fan running at full speed in G4520G6?

This option (Full Speed) is not available for G4520G6 system. Only you can do is set BIOS running as [Acoustic Mode] or [Performance mode] in BIOS setting.

The only difference between Acoustic mode and Performance mode is the SYS Air Inlet fan startup settings.
Please refer to the table below for details:

Note:
• Performance: Prioritizes cooling (Lower temperature, faster fan speed, louder noise).
• Acoustic: Prioritizes quietness (Slower fan speed, slightly higher temperature, quieter noise).
• "SYS Air Inlet" refers to the temperature (°C) at which the fan PWM changes.
• In Performance mode, the fan starts at higher speeds at the same temperature, improving cooling performance.
If you have any further questions, please apply for a ticket for more information.