Is it okay to just buy the same brand and speed ram
Unfortunately no one will be able to answer this. RAM is ONLY guaranteed in the form sold (from the same pack). Any mixed modules are not guaranteed compatibility. This is largely down to how delicate and the manufacturing process is, and how it frequently changes. Theoretically you could get 2 packs of the EXACT SAME make and model RAM, but from 2 different packs and they may not work at all with one another, because they are mixed modules.
Not saying it never works, just it's a gamble, and the only way to find out is my trying it, as even someone with the same system with the same model RAM may have a different outcome. Every system is different.
So all in all, the only way to know if they will work together is to try them. Ensuring timings/speeds etc. are the same is just a way of trying to minimise risk, but it absolutely does not eliminate it. Ultimately, being the exact same model by part number does not mean they're the exact same inside.
Resulting in 4+4+8GB, leaving the 4th slot for future upgrades?
Just remember if you are running a dual channel board, then the 3 modules will still only run in dual channel and in some circumstances this can degrade performance slightly as you will be running 3 modules in only 2 channels.
That and not all motherboards can run overly well with 4 DIMMs populated depending on the RAM model and speeds being run.
All in all, there's no damage that can be done, but the above are some risks you need to consider from a compatibility and performance perspective.
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