Help! I couldn't attend my primary school reunion because my father died. What do I say to thank them 4 invite?

[Breezy]

This is a bit confusing. You made it sound like your father sent you an email 4 years ago and you want to know if its too late to thank him. And should you PM him on Facebook after not speaking to him (your father) for 20 years.
I am assuming that you mean someone else though. The person who sent you the invitation to your reunion.......
Yes, certainly, send him a PM. "Dear (name), I am sorry I havent thanked you for your email of 4 years ago. Life tends to throw things in our path. I regret that I will not be able to attend the reunion. My father has passed away and there is so much that I have to take care of at this time. Thank you so much for thinking of me. Please stay in touch and I promise I will too. I would very much like to hear the details of the reunion when you get a chance. I will be there in spirit".