This is the first time me and my wife did cash top up into our respective CPF SA account via Retirement Sum Topping Up (RSTU).
I topped up $4,000 while my wife topped up $7,000.
Reason for topping up:
1) Tax Relief for 2020
2) 4% interest per annum (Power of Compounding)
3) Escalating toward Full Retirement Sum (FRS)
In total for myself, I had transferred $72,000 from my CPF OA to SA since 2017 (Link) + $4,000 from Cash to CPF SA.