Question Ram Upgrade

[Opethian181]

I have 2x 4gb Kingston Hyperx 1866 ram working at 1600 MHz with asus h97plus motherboard.However the ram is not manufactured anymore and I want to buy 2x 4gb more.I have 2 sockets free which ram should I buy considering compatibility issues?

 

[Dark Lord of Tech]

Mixing and matching memory is hit and miss. Even identical modules purchased at diferent times can be incompatible.

 

[Opethian181]

So I should buy 2x 8gb?