First you should always check :
- Maximum RAM supported by your Motherboard (further MoBo)
- The datasheet of the current ram -> Your ram's datasheet
As you can see your RAM runs on 1333 MHZ with CL9, CL value needs to be the same, on the installed ram and on the new ram, or the modules will not be acceptable by your board together and it will declare memory failure.(this is very important) , some will argue that not always this is the case, but keep in mind, normaly it results in no VIDEO signal (black screen).
If your MoBo supports lower/higher clock speeds(1333mhz) for ram, and if you combine it with lower/higher clock,
- The two modules will work effectively at the lower speed.
Your compatible RAM is KVR13N9S8/4 datasheet
Complications of uneven RAM settup.
With equal amounts of memory in both slots, memory can be
"interleaved" so that successive chunks of memory alternate slots.
That way, memory accesses get distributed to both slots almost
perfectly evenly, allowing their bandwidth to combine. With uneven
amounts, memory cannot be interleaved and has to be mapped first to
one stick and then to the other. A program that is accessing a large
contiguous chunk of memory will find almost all of its accesses going
to one stick, and there will be no bandwidth combination
This answer on SuperUser -> Source
In one word you loose the meaning of DUAL-CHANNEL, better to buy a KIT of 8GB ( 2x4GB on 1333MHZCL9/CL10) or 8GB KIT of 1600mhzCL11 but then you need to check either you need to O.C. your MoBo or update Firmware -> Your Mobo datasheet here