You are right. jBPM 4 only provide a form-plugin to integration freemarker web form in bpm-console. In Form plugin, bpm-console direct get process varaibles and task outcomes.
If you want to integartion your own web form, e.g. JSP, I will suggest you only record taskId and processInstanceId in your web form, maybe use hidden input field. Then you could invoke services provided by jBPM4 in you java class.
I think this is the simplest solution at this moment.