Upgrade from M1 iMac to Mac mini M4?

A Mac Studio Refurbished Mac - Mac Studio - Apple, not mini, is what you really want. Check it out? Then post to the Mac Studio forum with questions.

The M4 mini is not worth upgrading to - jury is still out on it. You’re better off with a Mac Studio, or even a refurbished M2 mini Refurbished Mac - Mac mini - Apple if budget is an issue, both field proven. Refurbished M2s won’t last for long so decide quickly. All the best.

The Geekbench website has performance benchmarks available in their “browser”.

If you’re near an Apple Store or an authorized reseller, visit and perform your own testing with the apps you use. This’ll probably give you the best feel for the performance.

There will always be better and/or faster and/or more capable gear becoming available, too. That’s the nature of the business.

Ask yourself these questions:

Are you unhappy with the performance of your current system?

Is it a stretch trying to your work done a timely manner if you are using your computer for business?

If the answer to those questions are a definitive yes, then it would be a time to upgrade.

If your answer is just "It would be nice to be a bit faster", then perhaps you may want to just wait until the answer two those questions is a definitive yes or wait until the machine is now longer supported with new macOS versions. In this case, spend the money that you would have spent on a computer on improving your work environment to accommodate your needs. FWIW, once you start working with two displays, you will likely not want to go back to a single display.