You can't mix DDR4 and DDR5 — period.

They’re physically incompatible, electrically incompatible, and the motherboard won’t even boot. If you have a soldered DDR4 stick, the socketed slot will also be DDR4, not DDR5. Same goes for the reverse: a soldered DDR5 stick means the socket is DDR5.

So total memory bandwidth is zero because the system doesn’t run. Memory standards don’t mix. If you need more RAM, buy a stick that matches what’s already in there — same generation, same speed if you want dual-channel to work right.