Upgrade RAM Memory for Mid 2011 27" iMac

Hi Friends,

i want to know wether is it acceptable to add more RAM memory to my iMac 2011 27" Mid model,

which is already having 2X2=4Gb Ram , now i want to add 4x4=8Gb Ram memory so total would be 12 GB where 16Gb is maximum capacity,

so does it going better my performance is it compatiable ? or do i need to change all the RAM slots with same 4 GB of Ram???

Suggest..

Regards

Jak