Your motherboard has only two ram slots.
If you want more ram, buy a 2 x 8gb kit and sell the old ram or keep it as a spare.
In theory, replacing one of the 4gb sticks with 8gb might give you 12gb.
8gb would run in dual channel mode and the odd 4gb in single channel mode.
The problem is that
Ram is sold in kits for a reason.
A motherboard must manage all the ram using the same specs of voltage, cas and speed.
The internal workings are designed for the capacity of the kit.
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.
What is your plan "B" if the new stick/s do not work?
Better to buy a single 2 x 8gb kit up front.