How much RAM do I need for Big Sur?

See the following from macOS Big Sur - Technical Specifications

General Requirements

• OS X 10.9 or later
• 4GB of memory
• 35.5GB available storage on macOS Sierra or later*
• Some features require an Apple ID; terms apply.
• Some features require a compatible internet service provider; fees may apply.

I'm just going from my own experience. My wife's older 2009 iMac had 8GB of RAM and she had to continually close windows and apps to keep functioning. I installed additional RAM and it now runs well (with High Sierra).

I suggest that running Big Sur or later will not perform well with only 4 GB of RAM. It will be very slow. I would strongly recommend 16 GB to function at a level you will find acceptable.

FWIW, I run Big Sur on a 2013 MacBook Air with 8GB without a problem.

Glad it's working for her. 😎

My 2012 MacBook Air with 4GB worked very well with High Sierra. I suspect that it had an SSD for memory swap made a difference (but don't have any facts).