Dual Channel generally requires the sticks to be the same size/speed. Not sure about that laptop specifically, but that's usually how it works.
Now, why it says it supports up to 12 if you can't remove the base 4gb, I'm not sure, unless there's 2 slots open? I know my laptop has 3 slots, 2 are easily accessible by popping a back cover off, the other one requires a bit more work.
The 4gb is soldered on the mother board. Also there is a empty slot where you can add more ram, but the mother board can only handle a max of 12gb in total. So a 8gb stick is the largest that empty slot can take.
You can run memory testing software to determine the specs of the onboard memory and then get a 4/8GB stick that has the same specs to get dual-channel performance, though a 4GB stick is your best bet.