You can mix RAM's of variable size but you should not mix RAMs of variable speed. If you 2 DIMMs of 4GB and 2 DIMMs of 8GB, you will have no problem. When you mix DIMMs of variable speed, data will be accessed with the speed which is lowest from all connected RAMs.
In your case, both 4GB and 8GB RAMs are of 1600MHz, so you will have no problem.