No, there are no guarantees when mixing DRAM, even of the same exact model, and as pointed out, here it's indicated a 4GB and an 8GB which are different models, different sizes, different chips, etc. There are numerous myths about DRAM that continue to float around, I address many of them in my facts and fiction article:
http://www.tomshardware.com/answers/id-2741495/ddr3-faqs-fiction.html