Can the 2021 IPad Pro M1 with thunderbolt 4 work with LG ultrafine 2016 model that only support thunderbolt 3 port?

The 2021 iPad Pro supports 4K video over Thunderbolt.

More detail here:

Charge and connect with the USB-C port on your iPad - Apple Support

USB-C displays

Your iPad uses the DisplayPort protocol to support connections to USB-C displays1 with these resolutions:

• Up to 4K for iPad Air (4th generation)
• Up to 5K for iPad Pro 11-inch (1st and 2nd generation) and iPad Pro 12.9-inch (3rd and 4th generation)
• Up to 6k for iPad Pro 11-inch (3rd generation) and iPad Pro 12.9-inch (5th generation) with Pro Display XDR

iPad Pro 11-inch (3rd generation) and iPad Pro 12.9-inch (5th generation) also support Thunderbolt displays.

To connect your iPad to high-resolution displays, use a USB-C cable that supports high-bandwidth connections, such as the cable included with the display, the Belkin USB-C to USB-C Cable, or the Apple Thunderbolt 3 (USB-C) Cable (sold separately).2

Your iPad also supports high-dynamic-range USB-C displays that use the HDR10 standard. You can switch between SDR and HDR modes by going to Settings > Display & Brightness and selecting your connected display. Your iPad matches the frame rate and dynamic range of currently playing content when you turn on Allow Display Mode Changes.

I suggest that you contact LG support team (indicating your exact monitor model number) and ask them.

Regards

Giulio

I have neither so this is not confirmation but this may help.... https://www.reddit.com/r/ipad/comments/nivkci/the_2016_thunderboltonly_lg_ultrafine_5k_does/