Mixing of RAM, especially DDR3 RAM often will not work or not work without issues even if of the same timings and voltage. It's always best to buy ONE matched, tested RAM Kit of the full quantity that you desire.
purchase memory that you have already. That will make 8 GB total. 2x2 + 2x2
The only problem is that I can not find any websites selling my current RAM, so I have to resort to other rams.
Well why dont you just buy a whole different 8GB set. Its only $45 nowadays. Mabye upgrade to 12GB for less than $80!
I guess I could wait in till black friday for some deals.