Just add the 2x8GB — 24GB is worth the risk

Yes, add the 2x8GB and keep the old 4GB sticks for 24GB total.

Mixing different capacity RAM sticks isn’t ideal, but it usually works fine. You might lose dual-channel mode for the 4GB sticks (they’ll run single-channel), but the extra 8GB of capacity will benefit you more than the slight speed hit in most real-world tasks — gaming, browsing, light video editing. If your board supports it, you’ll have 8GB + 4GB in each channel (asymmetric dual-channel), which is better than nothing.

The only real risk