I've done a lot of work with Mule ESB - development, design, running in production, etc. I would say an ESB would be overkill for you. If all you need to do is have one wrapper web service that calls 1-4 other web services, I would keep it simple and just implement it manually.
An ESB requires a server to run your projects on and there is a fairly large learning curve.
It would be easier to learn how to make the web service yourself compared to learning the ESB. It would also be easier to maintain a web service as opposed to an ESB.
However if you are going to have web services multiplying all over the place, then an ESB might make sense.