Mac Mini 2014, with 4GB RAM should I upgrade to Big Sur?

I recommend that you do not. Big Sur and Catalina should have a minimum of 8 GB of Ram to operate reasonably well. 16 GB is more realistic. It depends on what other apps you use which may or may not be RAM hogs. Finally, Big Sur itself uses more RAM than some of the older operating systems did.

You should be fine, if you are a light users and only run native macOS App's like Safari, Mail, Photos, etc...

You could experience some additional slowness, if you are running third party App's like MS Word, Chrome, etc...

I'm brand new to Mac (after having IOS for years) and picked up this Mac Mini quite cheap to have a play and see how I found it. El Capitan I found to be really slow so I decided to try updating - I went to Mojave as a test, as quite a few apps required 10.14 so thought this was a good place to start, and in all honesty it seems to run a little quicker than El Cap. I've decided not to go higher as I don't think the 4GB will handle any more. The stock programs take a few more seconds to open than I would expect, but once they are open they seem to run fine. I just wish you could upgrade the RAM, as I'm very happy with this machine otherwise.