Higher rate tax payer - pension tax relief

Hello I have just realised that when I was a higher rate tax payer, I could have claimed higher rate tax relief on my pension contributions. I was contributing to my workplace pension along with employer contributions. I was a higher rate tax payer until 08/04/21 when my employment ended. So far as I understand, I can only go back 4 tax years. This leaves 20/21 tax year. Looking at my pension provider summary for that year, there was no tax relief applied. Can you tell me: If 20/21 tax year is the earliest I can back date How to apply for a refund and what evidence or documentation I need to supply? Thank you

Hi
To claim Personal Pension relief, please write to us including evidence from your pension provider of payments made for the 20/21 tax year.
You will also need to confirm whether the amounts included are gross (before tax) or net (after tax).
Send your letter to
Pay As You Earn and Self Assessment,
HM Revenue and Customs,
BX9 1AS.