Using iMac 27” as screen for Mac mini M4 Pro

Buy a dedicated external display that fits your requirements and budget for the M4 Mac Mini Pro. An iMac is a computer, not a display, and is not supported as an external display for your new Apple Silicon Mac.

The display that I use with my M4 Mac Mini Pro is an LG 32UN880-B 32-in 4K Ultrafine unit with a Belkin Ultra-High speed HDMI cable. It is even better than my 2020 Core i7 iMac with its 5K display. Keep your 27-in iMac if you want it as a separate computer.

Using Apple's Screen Sharing, I can present my 27-inch iMac on my 32-inch Mac Mini display when needed or in reverse, bring up the Mac Mini on my iMac. Apple's Target Display Mode ended before Fall 2014 5K Mac models and is not compatible with subsequent Macs.

it's a rather ac power hungry display compared to a normal monitor and you can't use it as the only monitor the days of wired target display are long gone.

you can use it as a second display using third party options

Use iMac as monitor: 5 Solutions - 9to5Mac

but as stated you will need to get a primary monitor for your mac mini too, and performance of the said third party options would likely result in the imac not being suitable for stuff like games and movies and other things that requires performance and no lagging.