Really? Sounds like an authorized person who didn’t know what he/she was doing — or you were presented some add-ones that aren’t really optional and you declined.
Was the OWC Temp sensor installed? If not, there are a number of crapware fan controllers — install one of them. The down side is that you must control the temp and fans. The other down side is that the fans will spin full till the extension loads every time you boot. If the installer replaced the HDD with another and tried to make the OE sensor work, it usually doesn’t.
If the temp sensor was installed, another connector was not plugged in correctly. There are three and any one could be the problem. On a 2011, the sensor is an inline wiring harness—if part of that harness was designed to plug into the logic board, it’s the wrong one (2009–2010).
Ok. 4GB is not enough RAM for High Sierra. There’s plenty on eBay cheap. Buy some.
Now as to your question. If you have a backup that precedes the High Sierra installation, you can wipe the drive and install El Cap, then restore. You’ll need a USB boot drive or other external to do the install. If your backup is from High Sierra, your only option is a clean install but that is only really time consuming, not impossible.
High Sierra runs really well on a 2011 iMac with an SSD. It can work on an HDD but it isn’t recommended. If it’s an HDD formatted APFS, that’s wrong with High Sierra as it was possible only during the beta period.