The closest you will get to 'flawless' is a matched set of 4 sticks that are specifically recorded as compatible by the mobo manufacturer.
This is not an option for you, if you want to re-use your existing RAM. You could instead buy a matched pair of sticks of the same model as you currently have. While not a fully matched set, you should not have any problems.
You are not going to be able to buy a pair of Crucial sticks that are perfectly compatible with your existing Corsair one. However, if you buy a pair that are both supported by the mobo and have the same timings (e.g. 9-9-9-24) as the Corsair ones, you almost certainly won't have a problem. Usually the RAM timings are included in the RAM specs in the online stores.
And even if you put RAM with the same speeds but different timings, most modern motherboards can be configured to work in some sort of Auto mode that will find the best fit for your installed RAM. And almost certainly you will not notice any difference in real-world performance or reliability.
Summary: If it were me, I'd buy 2 more of the Corsair sticks or would get 2 x near identical Crucial sticks and would expect them to work without any problems, but if you want a placebo level of reassurance, buy a matched set of 4 sticks.
Note: it is a good idea when you get new RAM is to run MemTest86 overnight to tease out any potential problems - if you RAM passes such a test, you know you are in good order.