I've recently had this in January and from that sole bottle I had, it's time to drink up. It was already past its peak and on a slow downward slope. If you like older wines then you will enjoy it, if not then you may not.
Without having tasted the 1991 vintage, I would have given completely opposite general advice to Andy. My experience of Barca Velha is that it is a big and long lived wine with the ingredients and components to live for a long time.
But I guess that the only way you will know for sure is to open one of the four bottles you have!