Replace your 2x4GB with 2x8GB. Don’t add more 4GB sticks.

I would replace, not add.

Adding two more 4GB sticks gives you 16GB total, which sounds fine, but you’re gambling on the new sticks working well with your existing ones. Even if they’re the same model, different production batches can have different timings or voltage requirements. You might get random crashes or boot failures that are a nightmare to troubleshoot.

With a fresh 2x8GB kit, you know both sticks are matched and guaranteed to run in dual channel. Plus, you free up two slots for a future upgrade to 32GB if you ever need it. The cost difference is usually less than $20, and you can sell your old 2x4GB kit to cover part of it.

Don’t chase a few more gigs at the expense of stability.

Future-proof your build.