There's quite a few factors at play here - posting the make/model of your board will help.
Generally, mixing & matching RAM (speeds, brands, or even "kits" sold seperately) run the risk of not "playing nice" with one another). I've never personally had an issue with that myself, but there's many other who have.
You wouldn't be utilizing dual-channel (assuming your board supports) by mixing/matching, so in theory there would be no real difference between adding 1x8GB or 2x4GB.
In practise however, adding 2x4GB would be the better solution - especially if you can find the same make/model that you currently have. No guarantees though,