A common misconception about RAM is that you can put any RAM into any slot. ... If you have four RAM slots, always buy matched pairs of RAM (two sticks from the same company, same speed, and same capacity) for the best results.

Should you install RAM in pairs?

Memory doesn't have to be installed in pairs, but it is recommended in pretty much any modern motherboard as this is what enables Dual Channel mode which can (under some circumstances) dramatically increase performance.

Is it okay to put 4GB and 8GB RAM together?

In essence, the computer will be faster than when you had just one 4GB RAM stick, but it won't be as fast as having two RAM sticks of equal size. So, back to the “Can I use 4GB and 8GB ram together question”, yes you can, but a good school of thought recommends that you use two sticks of equal sizes.