I bought an iMac about a year ago and had the ram upgraded by the seller in 2 of the slots.(this has 4 slots total) They swapped 2 2GB out for 2 4 GB making the total 12 GB ram. They put in transcend brand ram. I do a lot of heavy editing, so I would like to upgrade the 2 other slots from 2 2GB to 2 4GB as well. The exact transcend part number that was put in before is no longer available. Could I put 2 new Crucial brand in with the 2 Transcend that are already in there? Or do I need to buy 4 so that there are all 4 the same? Thanks!