For CPF, after age 55, can top-up any amount in SA account to earn the 4% interest rate, eg $50k? And afterwhich can withdraw any amount anytime? Or have to top-up just below age 55?

Top ups direct to SA (before 55) / RA (after 55) via the Retirement Sum Top Up Scheme (RSTU), is set aside specifically for CPF Life.

Which means you would not be abe to withdraw it like a bank account. It will be converted into monthly payments from the Payout Eligibility Age for life.

Mandatory or Voluntary Contribution however would be available. But this is divided into the 3 accounts, OA, SA, MA and follows the contribution split of your age and has a $37740 annual cap. There's a way to increase that limit via another job, but I shall not expand on that.​​​