I am already running 4 monitors and before I invest in any additional hardware, I would like to know if this is possible.
Ah, I see now. So you have two monitors connected to USB-based graphics engine(s) and two monitors connected to the Intel HD Graphics engine.
Since only two monitors are connected to the Intel HD Graphics engine, you can connect an additional monitor through this engine. To do this, you will need to use a MST Hub on the DP output to support two monitors sharing it. MST Hubs are available from all sorts of places, including Amazon, and offer outputs in VGA, DP and/or HDMI formats. I am, for example, testing a MST Hub with two HDMI outputs currently. One port is supporting a 2160p@60Hz TV and the other a 1080p@60Hz TV (this pretty much maxes out the DP channel's bandwidth).
In order to get a 6th monitor connected, you can use another USB graphics engine, as you have previously, or you can add an internal/external NVIDIA or ATI HD Graphics engine and connect three monitors to it (replacing the USB graphics engine(s).
Hope this helps,