I wish I had heard about this a few months ago. I wanted to implement a way to create and connect streaming web services. I hacked up something then called webpipes (https://github.com/dnewcome/webpipes) using node.js. Unfortunately I haven't looked at it again since I first put it up on github. S4 looks a ton more advanced than anything I was envisioning, but I still think that something simple done in one of the evented servers like node.js (the S4 implementation looks to be Java) would be useful.