Race car has become a Go-Cart because of software.

Based on MacTracker, it appears that the most recent iMac with a 4.2 GHz quad-core Core i7 processor was the iMac (Retina 5K, 27-inch, 2017). That Mac cannot run anything later than Ventura.

If there were any other iMacs that had 4.2 GHz quad-core Core i7 processors, they were so old that they couldn't even run Ventura.

Whether you can update from Ventura or not will depend on your model / year of iMac.

For macOS 14 Sonoma & macOs 15 Sequoia you need at least a 2019 model.

https://support.apple.com/en-us/120282

For macOS 26 Tahoe you need a 2020 5K iMac or later.

https://support.apple.com/en-us/122867

For macOS 27 Golden Gate you need a 2021 M1 iMac or later.

https://support.apple.com/en-us/127255

Confirming the 4.2Gh i7 was unique to the 2017 5K iMac. I have one.

Historically, if a 2017 27" iMac was slow it was neither the OS nor RAM but storage or software conflicts.

Storage issues in that model were pretty much limited to:

—a too-full drive

— a failing/failed Fusion drive.

To the latter: Your model came standard with Apple's Fusion drive, a combo system consisting of a tiny solid-state drive hidden away behind one of the boards, and a larger 7200rpm SATA 6 mechanical drive in the main drive bay. The two were linked by software to act as one.

A pure SSD system was optional and expensive in 2017. My 1TB factory SSD was a US$700 upcharge!!

A healthy Fusion drive was much faster than a plain mechanical drive but slower than a pure SSD iMac of that year. The problem with Fusion manifested in two ways:

— due to unadvised or improper system software "wipes" and reinstalls, the two Fusion components "lost track" of each other. The drive has "split."

— one of the components, usually the mech hard drive, failed.

You can check for a split Fusion drive at home with instructions at the top of this Apple article:

How to fix a split Fusion Drive

We can get a more effective look at storage issues and any software interferences if you post an EtreCheck report. We can quickly and within the limitations of these forums help you determine what issues are at play without our playing a protracted game of "20 Questions" with you that could go on for days. 

EtreCheck Pro is available here: https://etrecheck.com/en/welcome.html

The free version will do nicely for this purpose, although the app is worthy of our financial support.

We can see hard data about your drive performance, software issues and interferences, and RAM usage. Etrecheck is the development of a long-serving and trusted ASC contributor. It is a reporting app, not a "fix-it” app, expressly for displaying information in these forums to help us help you remotely. It will not reveal any personal or secure information.

Please see this excellent user tip on how to post long text reports like EtreCheck's into a forum response:

https://discussions.apple.com/docs/DOC-250000211

NOTE: Please post the entire report unedited. There are subtle hints there that may seem trivial to a new EtreCheck user, but can speak volumes to those of us who have evaluated thousands of them.

What year model is the iMac...?

see > Identify your iMac model

The ability to upgrade the macOS, hinges on the year model of the iMac.

If your Mac is performing poorly, upgrading macOS would not fix it even if you could.

Defective hardware or software can be responsible for poor performance. To rule out the former, please determine if the same problems occur in "Safe Mode": Start up your Mac in safe mode - Apple Support.