Termination from job but never received any letter

Hi. I was terminated from my previous org. on performance ground 4 yrs back, though I was never an under performer, I used to be an average performer. I had some issues with my manager(but never complained to HR) and instead the manager involved the HR and tried to pressurized me for resignation saying my performance was not as expected, but I told them if that is the case put me on PIP(Performance Improvement plan) but instead they blocked my Login ID so that I couldn't mark my attendance. I had written so many mails to them to unblock my Id but I received no reply. When contacted HR, she told me that as my performance was not good that is why my ID is blocked and I need to resign otherwise they will terminate me. I told her to terminate me as , as per my knowledge every company has this PIP program for under performers just to give them a last chance to perform. But instead I was terminated. I had just received a mail that I have been terminated not even a termination letter mentioning the reason. Now, I have contacted HR 3 months back (dropped mails regarding the same,also visited once and met HR personally) asking for termination letter as I have again started looking for a job and I should at least have something documented why I had I been terminated, she said she could not find my termination letter but in their system I am terminated and she was not sure of the reason. she said I might have received the lowest appraisal ratings which might have resulted directly into termination. I have sent them reminders for the letter but never received any revert.I have one question, if my rating was poor then why they kept asking me for resignation for almost one and a half month when they could have directly handed over the termination letter to me. Please suggest if something can be done now as its too late, almost 4 years.