Depends on mobo, but generally for dual channel mose (up to a 10-15% performance increase over single channel) it's normally done in slots 1-3 or 2-4 from CPU
do you mean 2 and 4? or do you have to put them in 3 and 4. 3 and 4 will certainly lower performance, you wont have dual channel ram enabled. If you can, put them in 2 and 4, the same colored slots
Depends on mobo, but generally for dual channel mose (up to a 10-15% performance increase over single channel) it's normally done in slots 1-3 or 2-4 from CPU