x.js

httpProxy.createServer(function (req, res, proxy) {
    var host = req.headers.host
        , subdomain = host.split(".")[0]
        , port

    if (subdomain === "streams") {
        port = 4001
    } else {
        port = httpPort
    }

    proxy.proxyRequest(req, res, {
        host: 'localhost',
        port: port
    })
}).listen(httpProxyPort)