Having different sticks means you can't apply any default profiles such as XMP.
So you have to manually change any settings.
I suggest running Memtest86 to test stability, and if there are no errors reported (about 30 minutes for a full pass of 8GB) don't mess with the settings. You may want to overclock later but for now just make it stable and test long enough (at least a week).
Also, it MAY be the #2, #4 slots however you should verify the configuration in your motherboard manual.