Last year of employment before retirement

I want to maximize my pension tax free saving in my final year employment before retirement. Could you therefore confirm my understanding as described below is correct: Assume that my employment income for the year up to September is 20,000. If I put 16,000 pounds into a pension (20,000 - 20% of 20,000). I will then get 4000 pounds of a pension top up. I then will have effectively paid no income tax for the year in question. However my tax free allowance of approx 12,000 pounds as also been used up. If I then access my pension I will then pay income tax at 20% on all of it , assuming I stay within the 20% band.

Hi,
You will get £4000 tax relief on the pension contribution.
However, this payment does not exempt your salary from tax and anything above the personal allowances is still liable.
See guidance at:
Tax on your private pension contributions
Thank you.