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 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.

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).

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).