Since WCF supports two-way communications, this is technically possible. When client C1 sends a request through the channel, the server sends the requests to C2, C3 and C4 and waits until those clients respond. Once all four responded, the server sends the final response.
But are you sure you want to do that?
It doesn't seem you gain anything compared to more ordinary communication patterns.
If you think that you gain bandwidth, you're not (you'll even waste a lot doing all the additional requests and responses).
If you think that the server footprint will be reduced by avoiding to generate the same response to multiple clients again and again, this is a false assumption as well: it will increase (again, because of those additional requests and responses). Caching, on the other hand, will be more appropriate.
The drawback is that if any of the four clients is down, the other ones will never receive the response.