What ram should I buy for aspire E5 471?

I have acer aspire e14 ,e5 471, and I saw that i have 1/2 slots of ram in http://www.crucial.com/usa/en/scanview/B768F4E131D926C3  

Do i need to have 2 same ram installed for it to work? because I read something from other websites, If I need to have 2 same rams installed, what is the current ram installed in aspier e5 471? current ram is only 4gb, but is it okay if i buy same ram but 8gb? please help me...