Refund?

Am i able to get refunded for heirlooms from 4 years ago?

Nope. You have 24h for item misbuy, you can't return items 4 years later.

Am i able to get those items back when i had them on Neltharion?

Highly doubt it, Neltharion decommission has ended quite a few months back.
You can try contacting [email protected] but don't get your hopes up.