How to buy missing years of NI self employed, after retirement age.

I have recently reached retirement age, but am not claiming my state pension yet, until I have topped up my NI contributions (missing years) I am self employed, and currently earn just over the threshhold to pay class 2. Do I need to pay class 4? I can't find any where to find out how to do this. I have phoned the Future pension, and the pension service, and had no joy. Very confused, can someone please help. Thank you

Hi Andrew, 
Class 4 National Insurance does not count towards your State Pension.
When you are self -employed, its only Class 2 National Insurance that counts towards your State Pension.
You will only be charged Class 4 National Insurance once your Self Employment profits exceeds £12,570 for the current tax year.
If your profit is below £12,570 you will not be charged Class 4 NIC 
Please see guidance here: 
Self-employed National Insurance rates
Thank you.