Late 2014 iMac - use as screen

There is a workaround product called Luna Display, but even if you run a cable between the two Macs to help carry data, the connection will NOT be as good as one to a real monitor.

The Astropad site itself says that Luna Display is not recommended for high motion content applications like video editing or gaming, and that there may be lags and artifacts.

Target Display Mode went away when the 27” Late 2014 iMac - the first one with a 5K Retina screen - came out. This was likely due to technical issues with accepting extremely-high-data-rate 5K video.

Apple later added restrictions such that no current Mac supports using any iMac (even an old one) as a Target Display.

No. It is not a monitor. You can not use it as a display for your MacBook Pro.

It is not now, nor will it ever be possible.

Go buy an external monitor.

Please, does anyone know if this is now possible? I can't believe Apple still doesn't want users to utilise more than one screen in a working environment?

Yes you can: Turn Mac screen sharing on or off - Apple Support. Works fine on your late 2014 Mac.

Re: “I can't believe Apple still doesn't want users to utilise more than one screen in a working environment?”

How did you come to that conclusion? A M1 Pro MacBook Pro can drive two external monitors at the same time as the built-in screen. Apple even sells a 27” 5K Studio Display with a screen similar to the one that your iMac had. (Although given the way they priced it, many people settle for lesser but cheaper third-party 4K displays.)

CGJ64 wrote:

I meant in the sense that the money forked out to buy their quality devices for some of means longevity of use is highly sought

A Late 2014 iMac would be about 11 years old now. That's pretty old in computer terms.

That Mac originally ran Yosemite – and the end of the line for it is Big Sur. So it received six new major macOS versions. Big Sur is not one of the "most recent three" supported by vendors like Microsoft Adobe. It stopped being in that group two years ago, when Sonoma came out. It is still enough to run current versions of

• The Firefox Web browser
• The LibreOffice office suite

Any advice on what type of 4k Monitors I would need? How best to connect them?

Just about any monitor will work with the MacBook Pro, but I would suggest looking for one that has

• An IPS panel (so that if you view the monitor at an angle, there aren't huge color shifts)
• 100% or near-100% coverage of sRGB. If the manufacturer does not include sRGB coverage in their technical specifications, this is usually a sign that the monitor's basic color accuracy is not very goodl
• Optional: 100% or near-100% coverage of wide gamuts: DCI-P3 and Adobe RGB. (These overlap but are not the same. DCI-P3 is from the motion picture / 4K TV world, while Adobe RGB is from the print publishing one.)
• Modern inputs: one of more of DisplayPort, HDMI, or USB-C (DisplayPort). Many monitors offer two (or more) inputs of at least two different kinds. If a monitor supports USB-C input, a single cable can often carry video to the monitor, carry charging power to the computer, and hook up monitor hub ports at the same time.

The machine is 11 years old. It's a dinosaur. It had a good life. It's time to let it go.