I'm not sure if there are only 4 manufacturers of fly line or not. I suspect there may be some in the US and a bunch in China, Korea, etc.
I know that a number of top rod companies have their rods built by manufacturers overseas.
The thing is this: even if they are actually made overseas, the company provides the design for the materials and the design of the line itself - taper, weight, etc. As with most products we buy, fly lines are built with varying qualities of material and quality of manufacture.
I would guess that a very popular fly line, built by or to the specs of a long-time builder of fly lines could command a higher price for their product.
If you read the George Anderson fly rod shootouts, he prefers a particular, and expensive fly line to be used in rod comparisons. I would venture to guess he knows a bit about fly lines and their quality.
Now, that may only matter to a small percentage of the fishing public, but I think it supports price differentiation amongst fly lines.