The ServiceMixClient was not ported to SMX4. Here is a forum post for the similar discussion.
If you only need to access endpoints within the NMR, then here is a sample. You will need to install "nmr" feature.
However, if you need a client API for a remote connection, then I am afraid that there is not such a bundle yet.
Thanks for the example. I think this fact should be clearly stated in the doc, as I saw absolutely no documentation about that fact, which seems quite important as soon as you need access to the NMR.