Sup, gues you haven't been to the article section of illmuzik. Theres an article on flstudio that would've told you everything, but i'll explain. Fl Studio 4 features a Plugin called Rewired which basically opens an instance of Reason, and plays it through to the Fl Studio mixer, takes a lot of memory though, so make sure you pack up on that. To get th Rewired plugin, got to Channels -> Add One -> Rewired, and you can manage it from there, its not a complicated plugin, and before i forget to mention, it also allows you to open more than Fl Studio to work together, this is useful for composing melodies.
Hope I Helped