RAM not bought as a kit is not guaranteed to work together even if the exact same make and model. So you currently have a 2x4gb kit, those 2 sticks were sold as a kit guaranteeing both sticks will work with each other without issue. There is no guarantee those first 2 sticks will work with sticks from another 2x4gb kit. This is why RAM is sold in kits, if there were no risk of issue they would only sell single sticks.
For example I had a 2x4gb DDR3 1600 CL9 set and managed to pick up a second 2x4gb 1600 CL9 of exact same make and model really cheap (knowing the risk but had successfully mixed RAM before). When using all 4 sticks the system was very unstable, I could only get the system stable by lowering the speed, loosening the timings and over volting. I knew the risk and took a gamble. Other people who have done similar haven't got their systems stable however a lot of people have no problems at all.
Now my first hand experience is with mixing RAM is you can have issues but others are lucky and never have any problems. My experience of an issue was with DDR3 but I know the same applies to DDR4 too.