I think your question is:
Can I add another 2 x 4gb kit to get 16gb total?
The answer is maybe/maybe not.
Ram is sold in kits for a reason.
A motherboard must manage all the ram using the same specs of voltage, cas and speed.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards can be very sensitive to this.
That is why ram vendors will NOT support ram that is not bought in one kit.
You take your chances. You are more likely to succeed if the second kit matches the original in speed, voltage, and cas numbers.
You might need to increase the ram voltage.
Still, you should be prepared for failure. I might guess success possibility of about 50%
If you want a guarantee, sell your old ram and buy a new 2 x 8gb kit of supported ram.
Whatever you do, it is the motherboard that will only let you operate in dual channel mode, regardless if you have 2 or 4 sticks.