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.
Your odds of success are less than 100%. By how much, I do not know.
Are there any good statistics out there??
AMD processors are more tightly connected to ram and may not be as successful as modern intel chips.
You must consider the possibility that what you saved with a second 4gb may be wasted and you will need to buy a 8gb kit anyway.
You are on your own, but it is a good bet that you will succeed.
If the ram does not work, sometimes upping the voltage in the bios will do the job.