its all my ram running in dual channel?

i recently bought another 8gb of ram 2x4gb it isnt the same as the other 2 sticks but i needed more ram and it was cheap and has the same timings as the other 2 sticks. i opened up cpu z and it says channel # dual under the memory tab but when i go under the spd tab slot 1 and 2 say dual beside ranks but 3 and 4 say single. what does that mean? i can provide screen shots if needed