If I understand your problem correctly…
Set up a “Headphone Channel” under the studio tab of your VST connections. Assign the outputs for the headphone channel to your outputs 3 & 4.
In Cubase 7, make sure you see the control room in the Mix console, and that it’s set to the “control room” tab (not the meter tab). There are power buttons for both the “control room” channel and the “headphones” channel. Those power buttons control those channels separately, so you can turn off one and still hear the other. To mute your speakers, simply turn the power button off for the control room channel. Your headphones will still be playing.
Of course, with the control room (depending on how you’re using it and what you have set up) you can send all kinds of different things besides the stereo mix through both the control room and the headphones channel (and they can be playing different things simultaneously) so you’ll also need to make sure that the input sources of both channels are set to “mix”. 
BTW, if you want a step by step explanation of all the control room can do, feel free to check out my youtube video series here: