Using my M1 iMac as monitor for a Mac mini M4

Your 24" M1 iMac is too new to support Target Display Mode, and your M4 Mac mini is too new to support using any iMac as a Target Display.

See the bad news here: Use your iMac as a display with target display mode - Apple Support

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Your Macs are new enough that if your iMac is running Monterey or later, you could set it up as an AirPlay Receiver, and use it as an AirPlay Receiver for AirPlay to Mac. AirPlay is a lower-quality connection method … and the results may or may not be suitable for your purposes. And your Mac mini would need to have at least one real monitor for you to get to the controls to connect to an AirPlay display (like the iMac).

I would strongly recommend purchasing your hardware monitor(s) based on the assumption that they will be your only one(s).

Continuity features and requirements for Apple devices - Apple Support

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There are also third-party "workarounds" like Luna Display (Astropad) and Duet Display. These may involve some significant compromises – and again, you'd want a real hardware monitor for your first display.

You cannot use the iMac as a monitor for the Mac mini. iMac is a computer, not a monitor.

You will need a monitor to set up the Mac mini and any other optional features that you require.

You might be able to use Airplay:

Stream video and audio with AirPlay - Apple Support

depending on what you want to do.