How can I use a iPad Pro as a second screen?

The iPad User Guide is a rich (and often overlooked) source of information and guidance. Here is a direct link to the relevant section of the Guide:

Connect iPad to a display with a cable - Apple Support

If you are connecting your iPad to an HDMI port of the monitor, you'll need as suitable USB-C to HDMI Adapter. By example:

• USB-C Digital AV Multiport Adapter

https://store.apple.com/xc/product/MW5M3ZM/A

Alternatively, you might choose a USB-C Hub that incorporates an HMDI port. Here is one example from Satechi that is directly marketed by Apple:

https://store.apple.com/xc/product/HRDP2ZM/A

If you are referring to using a M4 iPad Pro as a second screen for a Mac, see:

Use an iPad as a second display for a Mac - Apple Support

Continuity features and requirements for Apple devices - Apple Support

Note that the M4 iPad Pro is not designed to be a hardware monitor for any other device.