Which slot for 1 stick of RAM

[thunderflash]

Hi I am building a PC for someone, they bought the components, but have bought 1 x 4gb stick of RAM instead of 2x2gb. Which slot should it go in on an Asus P8Z68-V LX please? no instructions in box (refurb)

Thanks

 

[thunderflash]

Thanks, as I said it wasnt me that bought the components, and he was going to get another 4gb later, but he only runs 32bit windows