William is correct. But I would like to expand on what he said.
Web service works in request/response style so when you invoke a method, it will send you the response of that method. If you must combine the responses you could write a proxy that will invoke 4 methods separately, intercept their responses and combine them.
Having said that, your design seems little bit awkward. Why would you want to combine the 4 responses in one XML? Are you sure you have designed your service correctly?