Can I use an old (mid 2011) iMac as a monitor with a new M4 Mac Mini?

Your Mac mini is too new to use another Mac in Target Display Mode:

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

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.

fredfromarlington wrote:

Thanks for the response. Which 24’ monitors should I look at? I don’t do gaming but want clear and solid resolution.

Things to look for in any monitor (whether 24" or larger):

• An IPS panel, for wide (178-degree) viewing angles. I believe that some VA panels may have wide angles, too. Monitors that use TN panels (because the vendor is being cheap, or is trying to get ultra-high refresh rates for gaming) may be prone to color shifts if you look at things a bit off-center.
• 100% or near-100% coverage of sRGB.
• [Optional] 100% or near-100% coverage of Adobe RGB, DCI-P3, or both.
• Modern inputs: DisplayPort, HDMI, USB-C (DisplayPort), or (for 5K/6K monitors) Thunderbolt. Many monitors provide two or more inputs, of two or more different types, which gives you more flexibility in how you connect them, either now, or to your next computer.
• A stand that is height-adjustable.
• A monitor that does not use Pulse Width Modulation (PWM) to handle dimming, or that has a high PWM flicker frequency. Some people are sensitive to PWM, and a low flicker frequency that's too high for them to perceive consciously will still give them headaches and eye strain. Monitor makers typically don't reveal anything about PWM, but there are some sites (like notebookcheck.net) that test for it and report results.

It looks like your 2011 iMac is a 21.5" iMac with a resolution of 1920x1080 pixels.

I absolutely would not bother with purchasing Luna Display to try to reuse that screen. You can buy an actual 24" 1920x1080 display for what the Luna Display would cost. It wouldn't be a very good or color-accurate one, but it would be much more convenient to use and have lower electricity costs.

For about $150 USD you could get a 24" 1920x1080 display with an IPS panel, and 100% / near-100% coverage of sRGB.

Or you could treat yourself to an upgrade – a 27" monitor with 2.5K, 4K, or 5K resolution.

I can see why people might use Luna Display to try to save a 27" 5K Retina iMac and use it as a second screen, but small 1920x1080 displays simply aren't as expensive or as difficult to replace as 27" 5K (5120x2880) ones.

You need to be more specific with the exact model number as BenQ makes multiple models 27" 4K displays (and a wide price range).

With that said, I have found LG displays to work quite well with Macs and at one time was a supplier of panels for iMacs so the display panel "look" is quite similar to iMacs.

No.

If you get a new Mac mini, then you need to use an external display.

see > Connect a display to Mac mini - Apple Support

In a word, nope! The MM needs a stand alone display, you cannot use the 2011 iMac (or any other computer for that matter) as an external display for the MM.