DDR4 RAM won't work on a DDR5 motherboard, and adapters are a scam.

No.

The physical notch is in a different place, the voltage regulation is different, and the memory controller on the CPU expects DDR5 signaling. There is no consumer adapter that converts DDR4 to DDR5 — anyone selling one is lying or selling a paperweight.

Even if some hypothetical adapter existed, it would introduce latency and instability. You’d spend more money on the adapter than you’d save keeping old RAM, and the performance would be garbage.

If you’re building a DDR5 system, sell your DDR4 sticks and buy new RAM. This is not where you cut corners.