4 years of NI contributions missing

I've recently realised as I logged into my Gov account for the first time (i'm over 60 and was used to receiving the paper versions), 4 years of NI contributions were missing from a previous employment. I contacted the employer directly, and they assured me through P60s all was paid from their side. However, on my HMRC online account it says "You did not make any contributions this year" across 4 years, which is mind numbing. I've got ACAS involved, but unfortunately not much could be done as its past the "3 month window" for claims and they assured me if I took employer to court i would be out of tens of thousands in costs. How can I sort this with HMRC directly, surely its a mistake on the system for whatever reason? 4 years of contributions cant simply vanish into thin air.. right?

Hi,
If you could send us copies of your P60s (or final payslip for each tax year if you don’t now have your P60s) of the years that your asking about (with a covering letter explaining that the contributions are missing) we’ll be able to trace for any missing contributions and put them onto your National Insurance account.  
Our postal address is:  HM Revenue & Customs, PT Operations North East England, Individual Caseworker, BX9 1AN.
Thankyou.