Using a M4 Macbook Pro with 27" Imac Retina 2016

It can’t.

Target Display Mode was deprecated years ago. Neither of those machines support it.

There are some software “solutions”, but the performance is not very useful.

Please note this advisory on the Luna Support home page:

The red emphasis is Luna's not mine. I read that as saying Luna can create a good place to park static windows and pallets, but never a primary display.

The costs of some of these third-party solutions could go quite a way toward the purchase of a proper free-standing external monitor that gives you ease of set-up, full video options, and uses less energy that a fully running iMac.

+2

Your best bet, is to get an apple or aftermarket external display to use with your MacBook Pro.

see > Use an external display with your MacBook Pro - Apple Support

+1

FWIW, there is also a hardware option called Luna. It may or may not suit your needs.

Just to add, There was no 2016 iMac. It's either a 2015 or 2017 iMac.