IMac (A1419) be used as a Monitor with an iMac Mini M4 Pro?

The very restrictive hardware and OS requirements for what you wish to do—Target Display Mode—are laid out in this Apple support article. Your Mini is far too new:

Use your iMac as a display with target display mode - Apple Support

There are third-party workarounds—I hesitate to call them "solutions"— but all are too laggy for a primary display.

Apple started the deprecation of Target Display Mode a decade ago and threw the last shovel of dirt in its grave in early 2020. TDM is dead tech.

Only a proper external monitor will deliver the responsiveness and display options you need for a primary display for your Mini.

To use the M4 Mini you'll need to get a 3rd party 27" or 32" 4k monitor with built in speakers. They can be had for around $100 to $350 depending on the size and brand. If you need a webcam that will have to be an additional add-on.

Not possible.

+1, nope it will not work.