You take your chances.
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.
This is more difficult when more sticks are involved.
If you add a 4gb stick, and it works, you will indeed have 12gb, 8 of which will be running in dual channel mode(flex mode) and the odd 4gb will run in single channel mode. All in all, a functional boost.
If the new stick is faster, it will only run at the speed of your original kit as well as the timings.
The problem is that it may not even run at all if the motherboard is unable to find a common set of speed, voltage and timings.
I put your odds of success at 80% mainly because of the native timing differences.
Brand is less important than the specs. Is there a 4gb stick of a different brand with the same specs as you currently have?
What is your plan B if the new stick does not work?
Could you return it?
Why not buy the game and see how it does with 8gb?
If you have a ssd, an increase in page faults due to a shortage of ram may not impact your performance.