What you have is a hierarchical clustering already specified (in your own data format convention), and you would like to use R's plotting facilities. This seems to be not easy. The only way I can see now to achieve this is to create an object such as that returned by hclust. It has attributes "merge", "height", "order", "labels", "method", "call", "dist.method" which are all fairly easy to understand. Someone already tried this: https://stat.ethz.ch/pipermail/r-help/2006-February/089170.html but apparently still had issues. What you could also try to do is to fill in a distance matrix with dummy values that are consistent with your clustering, then submit this to hclust. E.g.
a <- matrix(ncol=4,nrow=4, c(0,1,4,4,1,0,4,4,4,4,0,2,4,4,2,0))
b <- hclust(as.dist(a), method="single")
plot(b, hang=-1)
This could perhaps be useful.