How do I resolve Epson 4750 scanner error 21345 after updating iMac to sequoia 15.4.1?

I am sorry you are having scanner problems. If you will bear with me, I will offer some things to consider which may be useful. Whenever we install an operating system upgrade or some OS updates, printers and scanners will often stop working. Then a new OS compliant software must be installed for the scanner. What you can do is to thoroughly check Epson's web site for updated compliant software that works. If none is available, then you should ask when it will be released. If that doesn't work, you can try scanning using Apple's Image Capture app. It will scan, but is not full featured. Another alternative is to download and try the full featured scanner software from VueScan. You can try it for free. It will show a watermark until you purchase it. I bought it and can recommend it. I have no association with VueScan. Good luck.

VueScan Scanner Software for macOS Catalina, Windows 10, and Linux

I experienced the same problem after update to Sequoia. This fixed problem.

Go to System Settings > Privacy & Security > Local Network. Allow all Epson items to communicate with devices on you local network.

Hmm...

1) Give Image Capture a try.

Scan images in Image Capture on Mac - Apple Support

2) If that doesn't work, try reseting the Mac's Printing System.

Reset the printing system on your Mac to solve a problem - Apple Support

3) If that doesn't fix, then you might want to try the latest Epson Scan Driver.

https://epson.com/Support/Printers/All-In-Ones/ET-Series/Epson-ET-4750/s/SPT_C11CG19201?

FWIW we have an older Epson ET-2750 connected to our local Wi-Fi network. We have never had an issues Printing or Scanning with Image Capture from any of our Mac's running different macOS versions.

👍🏽