Mixing RAM (2x 4gb with 2x 4gb)

Solution
Well timings and speed are the same which gives you the best chance, however there is no way to know if they will play well together. If they use different chips (which they likely do) then they might not work together at all. Why not just buy another pair of the same, theres a far better chance of it working.

Rogue Leader

It's a trap!
Moderator
Well timings and speed are the same which gives you the best chance, however there is no way to know if they will play well together. If they use different chips (which they likely do) then they might not work together at all. Why not just buy another pair of the same, theres a far better chance of it working.
 
Solution
In theory they should be fine. But ram is super finicky you might find that you wont be able to run them at 9-9-9-24 Or that they wont run at all. Better if you can find another set of the crucial. Even better would be to get a 2x 8gb set to replace the 2x4gb set.