Matched pairs are more reliable.

Yes, mixing an identical second kit usually works, but a single 2x8GB kit is the safer bet.

Memory controllers are picky. Even two kits with the exact same model number can have subtle differences in internal chips or timings. That can lead to random crashes or failure to boot at higher speeds. Running four sticks also puts more stress on the memory controller than two sticks, especially on older or budget motherboards.

If you already own the 2x4GB kit and money is tight, buying another matching 2x4GB kit is worth trying — just buy from the same brand and speed, and be prepared to drop to 2666MHz or whatever your motherboard’s default is. But if you’re buying new, go with 2x8GB. It’s cheaper per gigabyte, leaves two slots open for future upgrades, and avoids the headache entirely.

Less hassle, more headroom. That’s the reliable choice.