Can I use an M1 MacBook Pro as monitor for M4 Mac mini?

The M1 MacBook Pro and M4 Mac mini are new enough that either can be an AirPlay Receiver, and act as an AirPlay display for the other. This feature is called AirPlay to Mac, and it might let you use the M1 MacBook Pro as an extra display for the M4 Mac mini. (You will still need at least one real hardware display.)

Continuity features and requirements for Apple devices - Apple Support

AirPlay is not as good as a hardware connection to a real monitor – so I would not advise you to purchase any Mac based on this feature. If you do happen to have two compatible Macs that you purchased for other reasons, then there is no cost to play with the feature (and nothing lost if the display quality is not suitable for your needs).

No, it’s a computer not a monitor - buy one!

There are third-party solutions such as Luna Display, but they are only suited for uses where there will not be fast motion and where lag time is not an issue, such as for tool palettes for an app. I think you'd be happiest just buying a new monitor.

Regards.