What you can do is create a plane surface which is significantly bigger than the bounding rectangle of all your points, then use the Surface Split component using all your lines as splitting geometry. It’ll give you a whole lotta triangles and one surface surrounding your line network. You have to find this surface (biggest area would be my first approach, but there are other ways), then get the edges, join all edges together, you now have two closed polylines, one outer boundary containing 4 segments and 5 points, and one inner boundary which is the one you want.
I think that’s the only way to do it in vanilla GH. Some plugin may provide a much easier way, but I do not know for sure or which one.