Hey there -
Brand+size does not matter, however the speed and timing of the ram does matter a lot. If they have the same specs then go for it -- if not then the ram will run at the speed of the slower ram.
Generally it is considered best practice to have all the same size stick, however you can get away with slots 1+3 and slots 2+4 matching.
However, if you have 8gb right now ... you could just get 2 more 4gb sticks and have 16 gb. Thats way more than you need. (Honestly 8gb is enough for 99% of users).