Abroad007

Hi I live abroad and spoke to HMRC on the phone who told me yes I should pay 4 years class 3 NI at least to make up the mininum amount pension. I completed the CF83 submission, was given how to format my ref number (my NI + IC + surname + first letter of name if less than 18 digits) so all OK. Was also given the bank account details to pay. My question is how do I speicify the 4 years that I want to pay for, as I only need to pay a small amount for those 4 years, versus the rest which are closer to 800 GBP... so if I just transfer a value totalling those 4 years, how will HMRC know which years I want to pay for. I cant pay online as this option is not available as others have mentioned, so bank transfer with ref number (without a way of informing speicifc years) looks like the only way.