There's nothing that stops you from building web services in B4J. Do note that it depends what your target environment is going to look like. If you have control over the server environment(i.e. you can install Java on it), you will be better served by jRDC if all you want to do is to expose DB data using queries to your B4A or B4i apps. If you intend to publish your web services as an API for a third party, then you should look into building a B4J web application, and build RESTful services for your users. B4J is very good with this.
Last point is, I personally do not think that B4J is ver well suited to building SOAP web services. So if this is required, you should perhaps use a different toolset. I may be wrong in this though and would love to know more!