Using an iMac 27 as a 2nd screen for an iMac 24 M4

You can't.

Reasonably modern iMacs can’t be used as a monitor for another computer.

Select iMac models (2009–2014) allowed to be used in Target Display Mode: Use your iMac as a display with target display mode - Apple Support, but the other Mac must be older as well (2019 or earlier, macOS Catalina or earlier) (not M4).

A1419

The A1419 model number has been in use for several hardware configurations and years (2012–2017), so unfortunately that is not precise enough in this case, even if the other computer could support it. Please find other identifiers on the chassis.

You may find a bit more information (like the year of introduction of the specific model) through the serial number (please don’t share serial numbers publicly), via: View coverage - AppleCare & warranty - Apple

TwinTurboo wrote:

I have been gifted an iMac 27 A1419, with keyboard, but no mouse. The owner is deceased, so I cannot get any passcodes. How can I set up the iMac 27 as a 2nd screen for my iMac 24 M4?

A1419 could be a 27" iMac released anywhere between Late 2012 and 2017.

The ones released between Late 2014 and 2017, which have 5K Retina screens, do not support Target Display Mode at all. The Late 2012 and Late 2013 ones support Target Display Mode, but Apple added restrictions that mean that no current Mac supports using any iMac as a Target Display. (Just making a Thunderbolt connection from a 24" M4 iMac to a 2012 or 2013 Mac would cost $80+ for an adapter and cable – and then you'd discover that you still could not use the iMac as a monitor. So there's no point in spending $80+ on that connection hardware.)

There are workarounds which could let you use the iMac as a second screen (e.g., Luna Display), but they may have significant drawbacks such as lags, artifacts, and unsuitability for high-motion uses like gaming and video editing. Also, they would require you to be able to get into the old iMac. That might be a bit difficult given that you say that the owner is deceased and that you cannot get any passcodes.

there are some third party solutions which will let you Use iMac as monitor: 5 Solutions - 9to5Mac

the performance is likely not up for using it for video or games though, and of cause a real 27" monitor would use a fraction of the AC power the imac 27" would

If your iMac 27 supports target display mode it’s worth a go.

I have a 2010 iMac 27 and followed the Apple instructions on connecting with target display mode to use it as a display.

Worked fine with my 2018 Mac book pro a before 2019 model with Sequoia 15.6 installed.

Also works fine with my M4 max Studio despite what the instructions say.

I do not need to log in to the IMac to use it a a display screen. It will do me until I buy a Studio display.

Your 2010 iMac supports input directly from DisplayPort sources. Most iMacs do not, including the majority of those that otherwise support Target display mode.

Unless the author’s iMac includes that particular spec, a TDM connection would not work.

You can find reference to the DisplayPort input support in the spec sheet.

iMac (27-inch, Mid 2010) - Technical Specifications - Apple Support

Hi,

Target Display Mode is old technology, so I think you cat't use old iMac for your new M4 iMac.

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

The iMac is an all-in-one computer. It does not have a video input port or support being used as only a display.

That would require one to be able to actually log into the Mac to be used as a display, which the OP apparently can't do.

Thanks for clarifying it, I am clearly fortunate to have the facility.

You’re welcome.