Flex mode works, but it's not ideal.

Probably, if your motherboard has four slots and supports it. You’ll end up with 4GB + 8GB in one channel and 4GB + 8GB in the other, which gives 12GB per channel total. That’s still dual-channel for the first 8GB per channel (the matched 4+4 pairs), and the remaining 4GB from each 8GB stick runs single-channel. Performance is fine for most people.

Check your motherboard manual. Some budget boards or older CPUs don’t support mixing capacities. If it works, you get 16GB total with a minor speed penalty. Not worth losing sleep over.

For the price of two 8GB sticks, you could just buy a 2x8GB kit and sell your old 2x4GB. That gives you true dual-channel and is cleaner. But if you already have the sticks, plug them in and see.