It should say on the spec sheet as upto 128gb memory which means they can support 4gb and they can also support 128gb. If they have listed it as supported it should work. But mixing ram sticks is not advised as it may lead to more BSOD or crashes.
What if my main objective is to increase the memory ,if it preformed as with out the additional rams it won't bother me that much.(the performance is actually good but it needed a lot of rams ) would matching them ( in size ) still be that important ?!!
rendering large projects using 3ds max , iam currently using teamGroup elite 2400 mhz (CL 16 -16-16-39 1.2 v) and i am think of is teamgroup elite plus 2400 0r 2666 (C16-16-16-39 1.2 v) which is Completely compatible with X99 chipset or team vulacn 16gb 2400 ( CL16-18-18-38 )