(1) If all were the same GB size and they were all from the same 4-stick matched set, they would be guaranteed to work. Yes?
Not necessarily guaranteed to
work in your system, as your system as a whole has to work with them, but those 4 modules from exactly the same pack
will be guaranteed to be compatible and work with one another, assuming the rest of your system is also compatible.
(2) If I populate all 4 slots, there may be a slight performance loss over just populating 2 slots, yes?
Potentially yes.
Btw, I already have the RAM in the slots and suffered no problems so far. I was just wondering about the performance loss.
That is probably fine then, not saying it never works, but it's a gamble. And in your case appears to work fine! So that's good.
It's still possible for issues to arise later (but less likely if they are working together normally at the moment, but still possible).
Performance loss will be hard to identify. If your system wasn't using the full capability of the original set of RAM, then adding new RAM will make no difference, it's just more cupboard space not being used. But can have a performance impact if it is running in dual channel across 4 modules.